I'm hearing this from multiple people on AT&T LTE devices, not specifically the One X. I have a buddy who has the Galaxy Note and he's telling me he has the same thing.. bars show, just data doesn't work at all. He has to put into airplane mode often.
welcome to the wonderful world of LTE . as a long time verizone customer i always enjoyed great service . since lte launched in my area it always seems to be something or another with the data no matter what phone i have . i have to turn off lte and just use the 3g alot of times . I really dont think LTE is ready for prime time .
Have not had this issue yet LTE going strong in dfw I will post if it stops working however I think you should do a restart or take the sim out and let it register again with the network.