It looks like the core Silverlight 3 tools are now available:
- Microsoft Expression Blend 3 + SketchFlow RC
- Microsoft® Silverlight™ 3 SDK
- Microsoft® Silverlight™ 3 Tools for Visual Studio 2008 SP1
- Deep Zoom Composer
Though the tools needed for development seem to be public, I’ve yet to see the end-user run-time; though I imagine we’ll see that in the release anticipated for tomorrow.
Time to have some fun. 
Update: Client run-time is now available.
Microsoft Expression Blend 3 + SketchFlow RC
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