My iMac has been running horrendously slow the past couple days, so I booted into the Snow Leopard CD and ran a disk repair and a permissions repair. Now I can't get past the spinner. It's been going for about 4 hours now with no shows of progress. Does any body have any ideas on what else I can try (I already tried doing the repairs again) or is there any way I can debug this?
Try Verbose Boot - Command-V while starting, it will show you what the systme is doing as it starts up instead of a pretty gray face. If you get anything like "disk0s2: I/O Error" you're looking at a failing hard drive. Go to the nearest Genius Bar or other AASP. If you get hung up with some other errors, try re-installing the system from the Snow Leopard DVD.
When you ran a disk verify (which, IIRC, you must do before you can run a disk repair,) what were the errors that came up? You may be looking at a dying hard-drive. Try to use the iMac as little as possible until we (or someone at Apple/with an Apple certification) can help you figure out what's going on.
If it'll boot into single-user mode (hold Cmd-S while booting) try repairing it with fsck (filesystem consistency check and interactive repair) -- "help fsck" when you get the prompt. Work carefully; you'll have root permissions!