Post-Migration MBP: won't boot, keeps restarting. Permissions problem?

Okay, long story short, I bought a new MBP, which we will call MBP2. MBP1 was fine, just old. I used the Migration Assistant to successfully transfer my files. Now, for some reason, MBP1 won't boot. It gives the grey loading screen for 2 minutes, restarts, grey loading, restarts, etc. I've booted in safe mode and fscked it, "HD is OK." When I boot from the install CD and try to repair permissions, it says "can't unmount." And yet, if I use target disk mode, it shows up on the desktop of my MBP2 just fine.

However, in target disk mode, Disk Utility has verify/repair disk available (which I did, "HD is OK)," but verify/repair permissions is greyed out. I unmounted it, but it's still greyed out. I know you can repair permissions from the command line on terminal, but can you repair permissions for one MBP on another MBP using the terminal?

Any other suggestions are welcome, of course - or just an explanation for why this might have happened! Migration Assistant presumably shouldn't kill your old computer.