I actually picked up a "Like New" Lumia 900 from the Amazon Marketplace for $250. Guy used it for a week and decided it wasn't for him. When you consider some iPhones and Androids are going for ~$500 used, that's not a bad deal.

Anyways, with that said, we are months if not weeks away from Windows Phone 8 being released. Some think it'll happen in September, others in October or November. If you can hold on, it might be worth waiting for. If not, the phones to have now are the Lumia 900, Samsung Focus S (based on the Android Galaxy hardware line), or the HTC Titan II. You honestly can't go wrong with one of those handsets, but you should definitely hit an AT&T store and handle them to see what you like.