As Silverlight RC0 and release come out, you will likely run into some breaking changes in your Beta 2 or older applications. The MSDN docs and SDK that is about to release has a breaking changes doc that lists most of these changes; however, due to these docs locking down, they are already slightly out of date!
Attached is a more complete up-to-date version of this document.
I'll update this doc as I dig up more tidbits.
Note: There are some breaking changes that apply specifically to Silverlight 1.0 applications. These are listed in the document but you can see them here as well.
Have a good weekend,