CPUs for the next XBOX are already being manufactured and distributed in devkits.
Honestly, I think a lot of this "we want to get X years out of our current console" from both sides is a smokescreen designed to gain some kind of edge in getting to market first.
It's a tough call, for me anyways.
On one hand I think "well the 360 has had it's time in the sun, but it's fast becoming obsolete", and on the other hand I see a game like RAGE come along and I'm left in awe.
There's no doubt that they could easily push the consoles we have for another 2-3yrs, wasn't it like 1 1/2 yrs ago when they came out with the slim version of the 360? It kinda doesn't make sense to develop a rehash of a console, spending a lot of money doing so, all to be made obsolete by a next gen system just a few yrs later.
Either way, I wouldn't want F4 to be on next gen. Far too many unknowns, and BGS already have enough problems when developing games on "stable" platforms.