It is very important that your applications are ready for Silverlight 3. If you have developed with the Beta bits you have to do a lot of changes to get ready for RTW. There exists a document changes.docx which I can not find at the moment. For that reason I decided to make a copy public on my private server.
If you run a Silverlight 2 application no action is necessary. The SL3 plugin can host SL2 applications. So there are no breaking changes between 2 and 3!
There are a set of bugs fixed in SL 3 related to SL 2. For compatibility reasons Silverlight plugin detects and change to quirks mode running SL 2 applications in a bug friendly mode.