I don't think they would go in that direction though; it undermines their entire reason for entering the console space in the first place: the digital living room. Between the teetering effort on the Vista side and the 360's strong leg-up on the Live/marketplace side, it would seem questionable to abandon that.
I think if anything, it goes back to theories of previous years - MS games play on a reference system that can be licensed out to a number of manufacturers, and also emulated/replicated on the desktop. I don't see MS going software-only for Nintendo and Sony - in fact that's probably the least likely scenario of all that could be envisioned here.
In terms of this particular move, I truly just think it has to do with internal dissent at Bungie, and a compromise deal that's been reached.
MS software tools for development and "lease" out the Live platform. It gets them back to their roots and makes money for them.
Also, the other option would be that MS is more content with picking and choosing games for exclusivity from various developers than owning studios outright.