I did an in place upgrade of Windows 7 x64 Ultimate over Vista x64 Ultimate SP2. I get random BSOD's (about two to three daily) accompanied by random freezes in certain games that did not occur on Vista (namely Left 4 Dead). My motherboard is an ASUS P5Q SE Plus, and from what I've seen, does not have drivers for Windows 7 (The website says they do, but when I download them and try to run then, they say they are "not compatible with this version of Windows".
As of now, the only thing I can pick out is the video drivers. I've tried like six versions of the video drivers. I'm running on.... 182.42 right now, having to lower my clocks on my video card (happened in Vista too, so that's not a 7 problem). They seem pretty stable.
But, as I have said before, I think it's the combination of the motherboard and the video card, and the fact that I did an in place upgrade.