Composite Application Guidance with updates for Silverlight 3 is now available on MSDN. Get it here

From project page

“
Note: This is a minor update to the February 2009 release. This release includes:

• Projects refreshed to target Silverlight 3.0
• Changes in DelegateCommand and CompositeCommand to no longer hold on to CanExecuteChange handlers.
• The TabRegionControlAdapter was updated to set the context for the Header item to retain the same behavior as Prism 2.0.

“