Just had the same thing happen here.
Client found update was taking too long on 10.7.2 Aluminum iMac, shut down.
Booting in verbose mode halts at "The System bootstrapper has crashed: Trace/BPT trap: 5"
It will boot into the Restore partition, and it will run DiskUtility. Ran Repir Disk, and Permission Repair found some things, but I've learned most are useless indicators.
Booted into Target Mode, ran Disk Warrior 4.4 on it, found some errors, but didn't resolve the boot issue.
I'm about to back up the user data, and blow it away and start over. :-(
Any other suggestions?
I found the solution. I got a mate to create a Lion recovery disk from his Mac then used that to basically re-install the OS. It left all my data intact.
Apple needs to make a Lion recovery or OS installation file available as a download for those of us that upgraded to Lion by download and don't have the OS disk.