The question of how long MS thinks the 360 will be selling for is quite different from the question of when the next Xbox will come out. Just look at PS2/PS3, the PS2 still sells well into the lifetime of the PS3, maybe Microsoft wants to try doing the same.
One thing which there's no reason to fear is another Xbox-style sudden discontinuation of the 360. The Xbox situation was very specific in nature, and outside Microsoft's control if I've heard it correctly (Nvidia owned the GPU design and didn't want to produce it anymore; no GPU, no console). This time Microsoft owns the whole console so no problems there.
Regarding the next Xbox, I don't think it will come earlier than 2011. It just doesn't make sense, they'll want the Wii to lose at least some steam before launching a console, which won't happen in such a short period. 2012 wouldn't surprise me either, anything later than that means that Microsoft had a radical change of plans.
My Mario Kart Wii friend code: 2707-1866-0957
