Mac won't boot: Way to mount drive and delete one file?

Solution 1:

You can boot into single user mode by holding Command + s at startup.

Single user mode starts you directly at the console, bypassing core Mac OS X startup processes. You should be able to remove or alter the offending file from there.

Solution 2:

Don't forget there's always Target Disk Mode, too. Start the Mac and hold down T until a Firewire icon appears - you now have the world's most expensive external hard drive. :) Plug into another Mac (or a Windows PC, maybe?) and you can access the hard drive on the machine in Target Disk Mode.

(Note that this doesn't work on Macs without Firewire. Mac Book Air users, you're bang out of luck.)