Just in case you haven't seen this clearly spelled out in the 1 gazillion other .NET-related news outlets, blogs, articles, papers, websites, forums, newsgroups, and so on - .NET 3.0 is out. The "Windows SDK for Windows Vista" is now available for download. Hmm, that name has obviously been approved by the Department of Redundancy Department. I wonder why they didn't just call it the Windows Vista SDK? It's not like there is a "MacOS SDK For Windows Vista", or a "FreeBSD SDK for Windows Vista." But I digress.
What is this Windows SDK for Windows Vista? According to the download page, The Windows SDK includes documentation, samples, and tools designed to help you develop Windows applications and libraries using both Win32® and .NET Framework 3.0 technologies targeting Windows Vista.
So, yes, that is what you want if you want the SDK for .NET Framework 3.0. The SDK will give you tools and sample code, to help you get started building .NET 3.0 apps. Like the .NET SDK 1.0, 1.1, and 2.0 before it, this is a free download; of course you need a licensed copy of Windows in order to use it. And you will need a text editor in order to create source code and what-not. You can use notepad; I use emacs myself.
If you use Microsoft's commercial tools, there are also .NET 3.0 add-ins for Visual Studio 2005. but that is a separate download.
If you just want to run apps built on .NET 3.0, you can also get just the .NET 3.0 Runtime, which Microsoft calls the .NET 3.0 Redistributable.
Oh, and by the way, .NET 3.0 is a strict superset of .NET 2.0. It is not an update of all things .NET, in the way that the .NET Framework 2.0 was to the .NET Framework 1.1. Instead, .NET 3.0 adds WCF, WF, and WPF to the .NET 2.0 class library and runtime.
Check it out.