This is the longest I've gone without blogging, but our PDC announcements have stuff way too cool to stay quiet about.
If you saw PDC, you've head that the CLR Debugging API, ICorDebug, will support dump-debugging. This enables any ICorDebug-based debugger (including Visual Studio and MDbg) to debug dump-files of .NET applications. The coolness goes well beyond that, but dump-debugging is just the easiest feature to describe.
This was not an overnight feature, and required some major architectural changes to be plumbed through the entire system. Specifically, when dump-debugging, there's no 'live' debuggee, so you can't rely on a helper-thread running in the debuggee process to service debugging requests anymore, so you need a completely different model.
Rick Byers has an excellent description of the ICorDebug re-architecture in CLR 4.0. He also describes some of the other advancements in the CLR Tools API space. Go read them.