I would recommend updating iTunes if you have not already and then doing a full restore of your iPhone and see if that does the trick. Sometimes the software can get corrupted and this can do the trick
If you restore from backup and it still does not work, Back up everything and restore as new phone sometimes the backup can be corrupt as well
I went to the apple store, and they said this is a very common probem with the new 2.0 software. They had me go to the nearest AT&T store and get a new sim card, and do a new activation on it. This seemed to help, but definetly didn't get rid of my problem.