Based on popular customer feedback, we have added a number of new extensibility points in Blend 3 (sorry, no support for plugins, yet, but who needs an officially supported extensibility model anyway? :) )

Over the next few weeks, I will provide more information on these, including interesting use cases that are already starting to pop up. Here is a quick listing of what we currently support:

1. Project and item templates: We support the same formats as Visual Studio, with some very minor modifications to suite our user experience. User project and item templates can be copied into C:\users\username\Documents\Expression\Blend 3\ProjectTemplates and \ItemTemplates respectively. We do not support wizards yet.
2. The Blend Asset Library
• Ability to register your custom controls. For example, to register a control library Foo.dll, all you need to do is to add the path to the control library as a value of the following registry key - HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Expression\Blend\v3.0\Toolbox\Silverlight\v3.0\MyCustomControlLibraryKey. If this were a WPF library, the key you would setup is HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Expression\Blend\v3.0\Toolbox\WPF\v3.5\MyCustomControlLibraryKey
• Adding an asset adds necessary assemblies to your project
• Support for custom control icons
• Setting custom properties on your control when they get instantiated - DefaultInitializer.
3. Design surface for custom controls
• Custom context menu commands
• Adorners
• Selection/object manipulation APIs
4. Property grid extensibliity  - same support as Blend 2, with a few minor additions here and there

Some other notes:
Blend 3 will share the same extensibility model as the next version of Visual Studio.

As we live in a world where there is greater and greater parity between WPF and Silverlight, we wanted the same for these APIs wherein you could use the same design time code to target controls for both platforms. To accomodate this, we had to make some minor breaking changes to the VS 2008 APIs (hopefully the changes will be very straightforward to adapt to) - I will try to post a number of samples to help out with this. By and large, the APIs remain the same as documented here (the documentation will be updated to reflect the breaking changes at a later date).

The APIs are not final yet, and are subject to change. Hopefully, we can keep the changes to a minimum.