I just did the OS 4.2 upgrade on my Ipad and now I get an error message that "no sim card" is installed. of course, ATT Ipad support is closed for the holidays and no one in customer service can help. Apple support is closed as well. I have restored the systerm a couple of times and done mulitple resets. I hate this stuff. (However, I hope you have a wonderful Thanksgiving!)
update: well, I restore it again, erased all data, and them did a total reset on the Ipad. after the 4th attempt my data card suddenly was recognized. I'm leaving the post here incase someone else has the same problem.