If there was a new Xbox in 2010 I think it would be a disaster.
If it is only a small leap from 360 to keep price down it will not have enough appeal aside from Halo 4 to encourage all but the hardcore to shift console. Not a pretty picture for MS.
It if it is a big leap and too expensive (considering that it looks like the economy of most English speaking (and Xbox friendly) countries are going to be in the toilet until at least 2011 it will not sell at all well and they will only get the real hardcore crowd again (assuming they have enough money).
2012 is earliest, unless the global economy seriously shifts, that I would expect a new console to hit the market and have a real chance of selling very well.
I know I won't consider a new console until 2012 earliest no matter what. I'm not getting into shortening console cycles just to help MS try and get out of the trap they've got themselves in with their console's demographic limitations.
Try to be reasonable... its easier than you think...