On Windows 7 x64 (AMD quad-core 16GB RAM); /never/ BSOD'ed until after the M9 install.
Two days of BSODs until I recalled the M9 font problems. Complete recovery after uninstalling both 8 and 9, then removing all Mathematica* fonts from c:\windows\fonts.
Reinstalled the new Mathematica 9 after stabilizing the system; no BSODs since.
Before recovery, during those two days of BSODs, Windows Safe Mode was my one stable point. Eventually I figured out that I could BSOD even that by opening c:\windows\fonts in Windows Explorer. I think the font "preview" triggered the BSOD. Of course, a plain-text dir listing at the cmd prompt did not BSOD.