After upgrading my machine to Vista yesterday, I went home and connected to it via Remote Desktop (from an XP client). It worked fine. However, when I came in to work this morning, I found both my screens blank and unresponsive - I got a mouse cursor and that's it. I could still connect to the machine through Remote Desktop and all my apps were still running fine, but sitting down at the machine, I got nothing. The only thing I could do was remotely restart the machine.
My instinct is that it's a video driver issue, but that's just a guess. More experimenting is required.