hmmm... i'm guessing that perhaps you don't have a boot prompt. if so, i've got some bad news adam. what you've got is referred to as the runaway level 9 problem. if you have a rescue disk, now is the time to use it, or if your installation cd has an option to get to the prompt, those are your only hopes. i had this problem a while back, and not a single guru i asked had anything useful to add. you need to edit your /etc/inittab file, but i'm guessing that you already know that. the trick is to get to the file.
in my unfortunate case, i had to do a clean re-install, which wasn't that bad, because i had just done a back-up. the only other real options are to try to install a small linux on another partition and then use that to fix the old system, but i've never tried that personally. if you find another way around, let me know.