Just a couple of little gotchas (which I always forget), I know it seems obvious.
If you are updating your Silverlight Beta 2 application to Silverlight 2 RC0, remember to update your version.
ASP.NET Silverlight Control Minimum Version
If you are using an asp.net page (with an ASP.NET Silverlight control), then make sure the minimum version of your silverlight control is updated to 2.0.30923 (RC0 version) rather than 2.0.30523 (beta 2 version).
HTML Page Object Type
If you are using the object tag for your silverlight application, make sure your type attribute is set to: type="application/x-silverlight-2", rather than type="application/x-silverlight-2-b2"