SOLVED: reinstalled ultrasnow. Rebooted. Working again.

I have an iPhone 3GS that was originally for AT&T, but was jailbroken and unlocked in February so I could use it outside of the country.

Everything was going fine (I'm on the 2degrees network in New Zealand), until today when I tried to install iRealSMS. It resprung but after that froze a few minutes later.

I ended up rebooting the phone by pressing the power button and home button down at the same time.

The phone is on OS 4.2.1 and now says the carrier is AT&T. So it has RE-Locked itself. GRRR. Any help? I'd rather not have to jailbreak the whole thing again.