A quick look at Apple's iPhone support forum seems to suggest that the latest firmware update for the iPhone (version 1.1.1, informally codenamed "Bricker") is toxic to both locked and unlocked handsets.
If I had an iPhone, there's no way I'd be installing the latest update on it (unless it was just to see what happened ...).
Unless you're feeling brave (or experimental)I'd give this update a wide berth for a while.
"But what about the ten security updates?" I hear you ask. They won't matter a jot if you trash your iPhone.