hmmm... when this happens, it's usually a file system corruption. but you say you've tried fsck (typing /sbin/fsck -y ) and it reports no problems? that then usually means the single user mode flag was incorrectly set to start you up in single user mode, which resetting the nvram should fix. it didn't, so it again points to a problem with your filesystem.
as a last resort, if you can borrow another mac, you can start it up in target disk mode and back up your home directory to it through the command line so you can at least save all your files.