Nokia has confirmed a software issue in the Lumia 900 that results in loss of data connectivity for the AT&T customers. Nokia says this is due to memory management issue that would be fixed by a software update and not due to a the phone hardware or the network.
Nokia announced the Lumia 900 with 4G LTE support at CES 2012 in January and began selling it in the U.S. for AT&T on April 8th for $99.99. They would offer a firmware update on April 16th to fix the bug or customers can get their phone replaced for an updated Lumia 900.
They would also offer $100 credit on AT&T bill for those who buys the phone between now and April 21st. The Nokia Lumia 900 is now on pre-order in UK that would go on sale soon.
[Update: Nokia has announced that the software update to fix the connectivity bug is now available, 3 days ahead of the scheduled date. To know more head to nokia.com/us-en/lumia900update]