Because they are engaging the same people who made the Cell processor once again. You only need the chip architects if you're developing a new chip and the kind of chips these guys develop are the kind suited to consoles. A new architecure = a new generation, by most definitions anyway.

It depends on the lead time between development starting and development kits being released to developers and from there how long developers need to finish the first games and the hardware guys to finish the hardware, finally from there a lead time for manufacturing.

Anything from as little as 18 months up to 36 months sound right?

I would say they have already started the process and the company they are working with is likely to be AMD for the GPU system, and maybe even the CPU. Now I don't know what the overall details of the system would be, but my guess is that its going to be heavily GPU compute biased to coincide with their development of direct X compute. The current Xbox 360 design is already GPU biased in terms of performance and I would expect that this would be even moreso the case for the generation coming up.

The question of who will make the CPU is a difficult one because they are Microsoft and there are so many different interests mixed up in everything. They have an abundance of programming talent and can litterally use any CPU they want, and they can litterally bring pretty much any CPU architecture into acceptance by supporting it with Windows if they so chose. However my current beliefs here centre around a low power CPU which can be used for embedded and low power handheld devices which can both run their mobile operating system and can scale up and offer a good perf/watt and perf/mm^2. Infact probably my best bet is an entire architecture which can be scaled between a handheld and a home console configuration and essentially kill two birds with one stone.