A developer-only release of Silverlight 3 Beta has been released.

Check out Welcome to Silverlight 3 Beta. This page gives you links to the Beta runtime, tools, documentation, samples, and summary of what's new in Silverlight 3 Beta.

Also, I'm copying down the "What's new in Silverlight 3 Beta" from that page here so that I can add links to new MSDN topics/API that correspond to some of the new features. See Welcome to Silverlight 3 Beta for the complete list including what's new for Blend 3.

Support for Higher Quality Video & Audio. With support for native H.264/Advanced Audio Coding (AAC) Audio, live and on-demand IIS7 Smooth Streaming, full HD (720p+) playback, and an extensible decoder pipeline, Silverlight 3 brings rich, full-screen, stutter-free media experiences to the desktop. New and enhanced media features in Silverlight 3 include:

• Live and on-demand true HD (720p+) Smooth Streaming. IIS Media Services (formerly IIS Media Pack), an integrated HTTP media delivery platform, features Smooth Streaming which dynamically detects and seamlessly switches, in real time, the video quality of a media file delivered to Silverlight based on local bandwidth and CPU conditions.
• More format choice. In addition to native support for VC-1/WMA, Silverlight 3 now offers users native support for MPEG-4-based H.264/AAC Audio, enabling content distributors to deliver high-quality content to a wide variety of computers and devices.
• True HD playback in full-screen. Leveraging graphics processor unit (GPU) hardware acceleration, Silverlight experiences can now be delivered in true full-screen HD (720p+).
• Extensible media format support. With the new Raw AV pipeline, Silverlight can easily support a wide variety of third-party codecs. Audio and video can be decoded outside the runtime and rendered in Silverlight, extending format support beyond the native codecs.
• Industry leading content protection. Silverlight DRM, Powered by PlayReady Content Protection enables protected in-browser experiences using AES encryption or Windows Media DRM.

Empowering Richer Experiences. Silverlight 3 contains new 3D graphics, animation features, hardware accelerated effects and text improvements that enable designers and developers to create next generation Web visuals. Additional features include:

• Perspective 3D Graphics. Silverlight 3 allows developers and designers to apply content to a 3D plane. Users can rotate or scale live content in space without writing any additional code. Other effects include creating a queue in 3D and transitions.
• Pixel Shader effects. These software based effects include blur and drop shadow. In addition, you can also write your own effect. Effects can be applied to any graphical content. An example would be to make a button appear depressed on rollover you could use a drop shadow effect on the pressed visual state.
• Bitmap Caching. Silverlight 3 dramatically improves the rendering performance of applications by allowing users to cache vector content, text and controls into bitmaps. This feature is useful for background content and for content which needs to scale without making changes to its internal appearance.
• New Bitmap API. With Silverlight 3, developers can now write pixels to a bitmap. Thus, they can build a photo editor to do red eye correction, perform edits on scanned documents or create specials effects for cached bitmaps from elements on the screen.
• Themed application support. Developers can now theme applications by applying styles to their Silverlight 3 applications and changing them at runtime. Additionally, developers can cascade styles by basing them on each other.
• Animation Effects (see Easing Functions): Silverlight 3 provides new effects such as spring and bounce. These make animation more natural. Developers can also now develop their own mathematical functions to describe an animation.
• Enhanced control skinning. Silverlight 3 provides easier skinning capabilities by keeping a common set of controls external from an application. This allows the sharing of styles and control skins between different applications.
• Improved text rendering & font support. Silverlight 3 allows far more efficient rendering and rapid animation of text. Applications also load faster by enabling the use of local fonts.

Improving Rich Internet Application Productivity. New features include:

• 60+ controls with source code : Silverlight 3 is packed with over 60 high-quality, fully skinnable and customizable out-of-the-box controls. There are new layout containers such as DockPanel, WrapPanel and Viewbox, and controls such as the AutoCompleteBox, TreeView, Label and Expander that ship in the SDK. You also get support for multi-page applications with navigation. You can download the Silverlight Toolkit to get additional controls; notably several charting controls  as well as nine professional designed themes and the source code that can be modified/recompiled or utilized as-is. Core control additions include more placement options for tooltips,  multiple selection in ListBox controls, and a SaveFileDialog making it easier to write files.
• Deep Linking. Silverlight 3 includes support for deep linking, which enables bookmarking a page within a RIA.
• Search Engine Optimization (SEO). Silverlight 3 enables users to solve the SEO-related challenges posed by RIAs. By utilizing business objects on the server, together with ASP.NET controls and site maps, users can automatically mirror database-driven RIA content into HTML that is easily indexed by the leading search engines.
• Enhanced Data Support Silverlight 3 delivers:
  • Element to Element binding : UI designers use binding between two UI properties to create compelling UI experiences. Silverlight now enables property binding to CLR objects and other UI components via XAML, for instance binding a slider value to the volume control of a media player.
  • Data Forms. The Data Form control provides support for layout of fields, validation, updating and paging through data.
  • New features for data validation which automatically catch incorrect input and warn the user with built-in validation controls.
  • Support for business objects on both client and server with n-Tier data support. Easily load, sort, filter and page data with added support for working with data. Includes a new built-in CollectionView to perform a set of complex operations against server side data. A new set of .NET RIA services supports these features on the server.
• Improved performance, through:
  • Application library caching, which reduces the size of applications by caching framework on the client in order to improve rendering performance.
  • Enhanced Deep Zoom, allows users to fluidly navigate through larger image collections by zooming.
  • Binary XML allows communication with the server to be compressed, greatly increasing the speed at which data can be exchanged.
  • Local Connection This feature allows communication between two Silverlight applications on the client-side without incurring a server roundtrip: for instance a chart in one control can communicate with a datagrid in another.

Advanced Accessibility Features. Silverlight 3 is the first browser plug-in to provide access to all system colors, allowing partially-sighted people to make changes such as high contrast color schemes for ease of readability by using familiar operating system controls.

Out-of-Browser Support. The new out-of-browser experience in Silverlight 3 enables users to place their favorite Silverlight applications directly onto their PC and Mac, with links on the desktop and start menu—all without the need to download an additional runtime or browser plug-in. Further, the new experience enables Silverlight applications to work whether the computer is connected to the Internet or not—a radical improvement to the traditional Web experience. Features include:

• Life outside the browser. Silverlight applications can now be installed to and run from the desktop as lightweight web companions. Thus, users can take their favorite Web applications with them, regardless of whether they are connected to the Internet or not.
• Desktop shortcuts and start menu support. Silverlight applications can be stored on any PC or Mac computer’s desktop with links in the start menu and applications folder, and so are available with one-click access.
• Safe and secure. Leveraging the security features of the .NET Framework, Silverlight applications run inside a secure sandbox with persistent isolated storage. These applications have most of the same security restrictions as traditional web apps and so can be trusted without security warnings or prompts, minimizing user interruptions.
• Smooth installation. Because Silverlight applications are stored in a local cache and do not require extra privileges to run, the installation process is quick and efficient.
• Auto-update. Upon launch, Silverlight applications can check for new versions on the server, and automatically update if one is found.
• Internet connectivity detection. Silverlight applications can now detect whether they have Internet connectivity and can react intelligently including caching a users’ data until their connection is restored.