Cellular voice, lots of signal, but no cellular data

I regularly take my unlocked iPhone 4s between the US and France, swapping my ATT and Orange (former France Telecom) SIM cards somewhere over the Atlantic.

But this time, after landing LAX, ATT was happy to show bars and do cellular phone calls, but there was no cellular data. WIFI works just fine.

I have tried the usual: turning off phone, resetting network settings. I tried my French SIM, no data roaming, but then it was also trying to talk to ATT.

Any ideas on how to tell whether this is an ATT or and iPhone issue? Yes, I know I can wait until I get back to France and try the Orange SIM.

My current theory is iPhone 4s knew that I had just bought a new battery from IFIXIT, and had a heart attack thinking about the damage I'd do, even though I haven't even opened up the new battery box yet. Others have suggested that it's a new Apple upgrade strategy.