Hmm, weird. I know filling in bad APN data on an iPhone will kill data for both AT&T and T-Mobile. It's how a coworker disabled data on his (not jailbroken) unlocked phone with T-Mobile. I tried a similar trick and found it worked for AT&T as well. It's also how I disabled data on my old RAZR with Cingular to avoid being dinged for accidentally hitting the stupid web button.