I'm (personally) starting to wonder what Microsoft's long term strategy really is, given that they don't seem to be interested in reducing the price of the XBox 360 and instead favour introducing a new model at the same price ...
I wouldn't be (that) suprised to see Microsoft introduce a follow-up system that was more powerful than the XBox 360, but based on the same basic architecture (therefore creating full backwards compatibility), and tried to do a "seemless" release of a next generation system; potentially designed so XBox 360 games could take advantage of the next system's additional processing power.