Apple - Repair disk from single user mode (fsck fails)
You can try to force fsck_hfs to rebuild the catalog by running
fsck_hfs -Rc -d /dev/disk0s2
or (if you want to avoid having to answer the prompts)
fsck_hfs -y -Rc -d /dev/disk0s2
Of course this will only fix logical damages and keep failing if the disk itself is damaged.
I found this page via Google and am dropping this here. Suffering the same "invalid node structure" and refusal to boot, I was able to get past the grey screen and boot to my desktop using the following steps:
Boot to single user mode (cmd + s on start)
Mount the failing hdd
/sbin/mount -uw /Exit single user mode by typing "exit" at prompt
(Not sure if this step is necessary.) Press the on/off button once.
Doing the above booted my Macbook (running Snow Leopard) to the desktop, bypassing the failing fsck check and allowing me to run an up-to-date time machine backup. YMMV but I hope this helps someone.
About three years ago, I ran into the exact same problem you're having. The issue is with the hard drive itself--nothing software related. Take it to the nearest/best Apple repair store and have them replace your hard drive. You can then restore all of your information from your backup. You do back up your computer, don't you?
(Note: if you forgot to back up your computer recently or otherwise cannot access your old files, the repair shop will most likely be able to recover most, if not all, of your data. If the HDD is really badly damaged, however, you're best bet is to send it to DriveSavers for a cleanroom disassembly and data recovery. If you choose that option, however, be prepared to spend several hundred dollars.)