A number of early Lumia 900 buyers on AT&T are reporting that their HSPA and LTE data connections are dying, rendering the phone significantly disabled (voice and text appear to be unaffected). At least some of those buyers have been advised by AT&T to try a series of restarts, master resets, and SIM pulls; the technique has worked in some cases, though it appears that the fix may only be temporary.
Interestingly, we ran into this issue on one of our review units last week, though it was our understanding at the time that it was a problem specifically with provisioning of those units in AT&T's systems and wouldn't have an effect on retail units. It now appears that may not be the case. We've reached out to AT&T for comment — stay tuned for updates.