The .NET Compact Framework 3.5 Redistributable includes everything you need to test .NET Compact Framework 1.0, 2.0 and 3.5 applications.

http://msdn2.microsoft.com/en-us/netframework/aa497273.aspx

Here is some other great new content on the DevCenter that has been posted to support this launch:

• .NET Compact Framework 3.5 Run-Time Breaking Changes
  http://msdn2.microsoft.com/en-us/netframework/bb986636.aspx
  Some new behaviors have been implemented which provide previous behavior compatibility to support applications built with previous versions of the .NET Compact Framework.
  
• What's New in the .NET Compact Framework 3.5
  http://msdn2.microsoft.com/en-us/library/bb397835.aspx