Already downloaded VS 2008 RTM? Great!
Doing some Silverlight 1.1 development? Great too!
However, the combination of both is not yet possible. At this moment, you can't use the Silverlight 1.1 Tools Alpha for Visual Studio yet with the RTM release. Here's what Scott Guthrie says on this subject:
Two popular add-ins to Visual Studio are not yet available to download for the final VS 2008 release. These are the Silverlight 1.1 Tools Alpha for Visual Studio and the Web Deployment Project add-in for Visual Studio. Our hope is to post updates to both of them to work with the final VS 2008 release in the next two weeks. If you are doing Silverlight 1.1 development using VS 2008 Beta2 you'll want to stick with with VS 2008 Beta2 until this updated Silverlight Tools Add-In is available.
So, the message is clear, don't go uninstalling VS 2008 beta 2 just yet, as you won't be able to work on Silverlight projects anymore.