Today's release candidate includes a ton of bug fix and some significant performance optimization work.

Today's release candidate also includes a number of final API tweaks designed to fix differences between Silverlight and the full .NET Framework.  Most of these changes are relatively small (order of parameters, renames of methods/properties, movement of types across namespaces, etc) although there are a number of them.  You can read this blog post and download this document to get a listing of the known API breaking changes made from the Beta2 release. 

We have updated the styles of the controls shipped with Silverlight, and have also modified some of the state groups and control template names they use.  When upgrading from Beta2 you might find it useful to temporarily remove any custom style templates you've defined, and get your application functionality working using the RC first - and then after that works add back in the styles one style definition at a time to catch any rename/behavior change issues with them.

If you find yourself stuck with an question/issue moving from Beta2 to the RC, please report it on the www.silverlight.net forums (Silverlight team members will be on there helping folks).  If after a day or two you aren't getting an answer please send me email (scottgu@microsoft.com) and I can help or connect you with someone who knows the answer.