I'm posting this in the hopes that someone searching for the same issue I had will find it and it'll help them. There's already the broader "Bluescreen" thread on this topic, but I ran into what at least seems to be a variant of Logitech's Control Center driver package killing Leopard. I carefully prepped my MBP for Leopard, but had forgotten that I'd installed LCC a while back to use my desktop Logitech mouse temporarily. I'd removed the prefpane, but it was still launching the daemon via Login Items. Now, surprisingly enough, this DIDN'T kill Leopard--it installed and ran fine. But then I noticed that it was launching the daemon, and wanted to make sure everything was uninstalled. Except I hadn't saved the installer for that version, so no uninstaller was available. Ok, I'll just go grab the most recent one... which doesn't include an uninstaller. Oh well, I think, I'll just upgrade with that then uninstall IT instead. Except when I installed LCC (2.3.1) and restarted as required, Leopard wouldn't boot. This was NOT the bluescreen bug--it would sit at the grey spinner for a couple minutes, then restart, and repeat endlessly. Looking at the verbose mode, it was stalling during initializing bluetooth for some reason. What makes this different is that I couldn't find any Unsanity stuff installed, and removing everything I could find via single user mode didn't help, and I kept getting an unfixable error on fsck. Finally, I ran TechTool Pro on the drive, let it fix some directory stuff, and THEN it would boot. At that point (thanks to the web) I knew to look for the Logitech uninstaller app in /Library/Application Support/ (which is ridiculous since they don't TELL you that's where it is), I uninstalled it, and all was well. I would think that the disk corruption was unrelated, except I'd restarted several times (installing drivers for other things) and it worked fine. I'm assuming that LCC caused a crash on the next reboot, which then caused corruption that prevented a boot even when LCC stuff was manually removed. Could be a bizarre coincidence, but given the other problems I've had with LCC, I'm not so sure. Bottom line is that even if your problem is "hanging at the spinner" rather than "blue screen at boot", it might still be LCC related. Second, if removing the LCC stuff via terminal doesn't work, try doing a disk check with something other than fsck. Further, if you need to find the uninstaller and it's not in /Library/Application Support you should probably dig around in the installer package for it rather than upgrading and uninstalling--hopefully, anyway, that will work. On an unrelated note, I love Logitech's hardware, but I refuse to buy anything else from them, since (though I'm not averse to 3rd party drivers) Logitech's Mac drivers are so willfully bad (HOW many years did it take them to fix the "ridiculous pointer speed on every reboot" bug?) I refuse to support them.