I am getting the following message when trying to create a new Silverlight application in VS2010RC:
This application was created for an expired beta release of Silverlight. Please contact the owner of this application and have them upgrade their application using an official release of Silverlight.
What do I need to do to resolve this issue?
The final releases of both Visual Studio 2010 and Silverlight 4 are available, so I suggest you "upgrade".
Now that Visual Studio 2010 & .NET 4.0 been officially released, the beta and RC versions will start refusing to work and instead posting messages like you are getting.
To avoid the error, you will probably need to install the RTM (Release-to-Manufacturing) versions of the Silverlight 4 SDK and the Silverlight Tools. At worst, you would need to upgrade .NET 4 and Visual Studio 2010 as well.
The Silverlight 4 Tools page may be the first place to find the latest versions. There is a link to download the Silverlight 4 SDK there, but I'm not sure if that's really the RTM version.
Also, note that often Microsoft expects you to fully uninstall the beta or RC versions of their software before installing the final version.