2.0 and 3.0 both use the 2.0 CLR (common language runtime). The only difference is addition of Windows Communication Foundation, Windows Presentation Foundation, and Workflow components in 3.0.
3.5 will be shipped with the next Visual Studio and the most dominent change is the addition of LINQ (language integrated query) technologies. There will also be new framework classes (for example, new encryption classes designed to utilize CNG). The languages themselves will probably have more significant changes than the framework. For a complete list of changes, you can view the Beta documents.
