The Lumia 900 (both black and cyan) has gone out of stock according to the product listing at the online AT&T store. Nokia has recently confirmed a memory management issue with the Lumia 900 running on AT&T that would kill the data connection, rendering the Windows Phone almost useless for owners while on the go.
Since the announcement was made Nokia has offered $100 on the bill of affected AT&T and Rogers customers and has also begun rolling out the firmware fix for the connectivity bug days early. There have been reports that the U.S. carrier has been pulling handsets from shelves in favour of models with the update already installed. We haven't heard anything official but this could explain why the handset is temporarily "out of stock".