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 the highlighted features, but it was still great to see it working on a real, honest-to-goodness device. What other secrets this software holds will remain that way for now, but we can certainly look forward to an action-packed summer full of interesting revelations.

Update: We were able to snap a few extra images of this very same Lumia 900, but we weren't allowed to touch the screen or interact with the software in any way, since the build we saw of Windows Phone 7.8 is just a special version highlighting the Start screen and is nowhere near final production quality. For what it's worth, we had the opportunity to view a demo of the tiles being resized, and we couldn't detect any flaws in its execution. Check out the full gallery of precious pics below!