We've heard from AT&T that sales of the Nokia Lumia 900 are "exceeding expectations," though it isn't saying exactly how many have been sold so far. If AT&T's site is to be believed, the Lumia 900 is selling so well that it's out of stock. We've heard similar claims from some AT&T retail stores recently, as well. While that certainly helps back up AT&T's claims, it's worth noting that the handset is still available from Amazon Wireless. The fact that the Lumia 900 is out-of-stock at AT&T might be related to good sales, but it's also possible that AT&T is removing the phone from its online store due to the data connection problems that have plagued this handset's launch. That said, with a software update rolling out to fix those data connection issues, hopefully AT&T will have the Lumia 900 back in stock with the update applied soon.