Today I've upgraded my 64-bit Windows Vista desktop to Windows 7 Beta 1. However, after a seemingly succesful in-place upgrade, I ended up in a black screen listing the Windows 7 build number (7000) in the lower right corner. Apart from a fully functional default mouse pointer, nothing happened, even after waiting for 30 minutes as was suggested by this post. CTRL-ALT-DEL did work however, making me believe it was the setup that was stuck. Fortunately, the setup created a special boot menu entry that performs a complete rollback that actually works.
Obviously I did not like that all, so I continued my research for finding a solution. Somehow, some arbritrary post reminded me of the fact that I have both my TFT monitor and my Panasonic Plasma TV connected to my videocard. And what do you know! That solved it!
So my advice to you is: make sure you disconnect any secondary monitors or TVs before performing a Windows 7 install or upgrade.