It may not be the mysterious prototype Joe Belfiore showed off at today's Windows Phone Developer Summit keynote, but it's a nice treat nonetheless: we had the opportunity to sneak a peek at an AT&T-branded Nokia Lumia 900 running Windows Phone 7.8, complete with the brand new Start screen. Because it's at such early stages in the development game, we weren't able to get our own grubby hands on the device and play around with any of
