I've been watching the MS Tech Ed North America sessions on channel9 and I came across a video that seems to show a version of Microsoft Visual Studio 2010 running inside MacOS X (Not in a virtual machine). It turns out this is VMWare Fusion 3. I am impressed at the level of integration with the Mac experience.
Look around 11:30. Notice the Mac OS menu bar clearly says "Microsoft Visual Studio 2010" as the running application. There's even a Visual Studio icon on the dock.