The Xbox 720: We all know it's coming, but it hasn't been announced. It has, though, been "kind of" confirmed by a Microsoft general manager.
In an interview with The Verge, Brian Hall General Manager of Windows Live, said that the new Xbox will be released "in time" for a spate of new Microsoft products, including Windows 8 and the Windows phone. You'll find the entire, relevant quote beneath the "Read More" tag.
“We’ve had Hotmail and operated Hotmail for about sixteen years, we obviously have Exchange, and Outlook, that people use at work. We just decided it was time to do something new and bring the best from each of those and put them together and release it right in time for the new wave of products that we could have coming out with Windows 8 with the new version of Office with the New Windows phone and the new Xbox.”
We don't really have any other information, but right now, this is the closest thing to a confirmation of the next Xbox that has come from Microsoft.