Ok, I have to say something else.
In a lot of ways, it makes sense to think that MS hopes that the 360 Kinect combo can be their Wii (GC Wiimote). If it really takes off in 2010-2011, why would they release a new console? Just because of some idea about how long a gen should last? Keeping up with technology?
Again, look. at. the. Wii.
There is no graphics breakthrough that we know about that makes the next gen worth it. Over 70 million people proved that already this gen by buying the Wii.
Who is demanding this next gen? Consoles are still pricey compared to previous gens (PS3 just now cost what the PS2 did at launch), the console profit margins are still slim (no pun intended) and game development still costly.
End of 2013 at the earliest for the next Xbox. That gives 3 full unchallenged holiday seasons for Kinect. Considering the investment, it'd be crazy to expect anything less.