A few days ago, when Nokia announced the Lumia 900 for AT&T, it said the smartphone would be released "in the coming months.". Now, thanks to the official Nokia Developer Newsletter (see the screenshot below), we know that the new handset should be out in March. Of course, that's when AT&T will launch it in the US only, as there's still no word if or when we're going to see the Lumia 900 on other carriers / in other markets.
The Lumia 900 is Nokia's third Windows Phone Mango handset, and is pretty much a larger Lumia 800 with LTE connectivity.
Unwired View, Nokia Lumia 900 to be launched by AT&T in March