So finally AT&T has put up a coming soon page for Lumia 900. There is no release date given on the page. But again grapevine has it that Lumia 900 has already reached to AT&T stores and the stores employees will be trained on windows phone OS and Lumia 900 in coming days. After the training is over  Lumia 900 will see release on 9th April,yes you read it right, this is the day when you will be able to purchase one.

AT&T Lumia 900 page