The reason we haven't seen a new Xbox is because, well...they don't really need one. It would be superfluous - with Halo and Call of Duty pumping out consistent semi-annual releases, they're really starting to dig into the mainstream market, something we didn't really see happen with the original Xbox or PS3, and they've just flooded the gaming market with an endless supply of customers by doing so.
If we do see a new console, I think we can rest assured it's not coming for a few more years and that the only reason we're seeing it is that Microsoft has finally come to the realization that current-gen console hardware is getting crushed by the newer PC hardware. Even then, i get the feeling they don't particularly care, nor do they need to.
And what else, at this point, can they really offer as an innovation in the console/general video game market? Windows 8 is actually trying to do some neat stuff with its Metro interface, but a new interface is just not going to cut it when it comes to releasing a whole new console. Motion Control as a gaming infatuation has come and gone, largely proving that it doesn't do much for a game at all, and I just don't see them thinking too far outside the box beyond that. Project Natal (I still call it that, god) was more or less a failure, expanding on that would not only be redundant, it would be pointless - unless they want to build it right in to every system, I just don't see it being that useful or worth the investment.
My two cents.