Me too. But I think I found the technic to unjailbreak/restore it.

1) I use latest d/l of redsn0w. Jailbreak it again (even though it is already jail-broken). This is to update all the jail-broken files in your iPhone first.

2) Use redsn0w to Restore ipsw to 5.1.1 (not latest) -> if step 1 above is done, no problem here. Very smooth and straight-forward.
If step 1 not done, got many errors come out and fail.

Took about 20 minutes for the phone to completely restore.