One of the biggest complaints of heard the the past from developers when new versions of Visual Studio comes out is that the can not take advantage of the new IDE because their company/project is still using the previous version of the .NET Framework.
A major step that was taken in Visual Studio was to separate the IDE from the framework. This means you can use the goodness of VS2008 on existing 2.0 projects. Brad Abrams links to the ASP.NET Ajax 2.0 templates that you can install in VS2008 or create new web projects targeting 2.0.
Don't let framework adoption rates prevent you from using the great new features in VS2008!