Details here.  As far as I can tell from the docs, they removed a lot of things, made a bunch of things private or internal and changed the signatures of some things here and there.  I can only hope that I don't have to do massive rewrites of all my Silverlight stuff.  Does anyone from Microsoft want to tell us about any new features that were added?

UPDATE: New features and breaking changes.

UPDATE 2:  I've updated Caliburn and my super secret other project without any hitches.  Good job Microsoft!  The new control templates and rendering look much better too!