Tim has done a great job of explaining in lots of detail what you should be doing as a developer with RC0 in preparation for the final RTW.
However I feel a simplification may help - then you can read what Tim has to say 🙂
If you have any users of your Silverlight Beta 2 code then there is a high chance that you Beta 2 code will stop working on their machines when we do our final release of Silverlight.
Hence… you need to test on RC0 now and be ready for the final release. Thanks all.