In the Holiday times i try the brand new Win 7 Beta 1 – 7000.0.081212-1400_client_en-us_Ultimate-GB1CULFRE_EN_DVD.ISO and for sure that i want that virtualized, so i use for some time the Sun VirtualBox to host my virtualized development environment. In the new Beta 1 are many fixes, features and updates of the previous version from PDC2008, look to entire list here: http://windowsteamblog.com/
on installing virtual machine additions i got the message "This Operating system is not yet supported.“ but just using simple Device Manager , driver update lookup to M additions folder i was able to use the Network card and extended video adapter.
Btw. Windows 7 developer guide it is also released http://code.msdn.microsoft.com/Win7DeveloperGuide cool things inside, Windows Biometric Framework (WBF) and Direct3D 10.1 Command Remoting and many others cool new stuff.