I don't really see the need for MS to get into chip design but I could see MS buying a fab or two. Licensing technologies is a lot cheaper then getting someone else to manufacture that same technologies. MS owning its own fab would do alot to reduce the cost of future consoles then having someone else manufacture MS designed chips.
The over whelming reason why you see fabless companies is cost. I seriously doubt ATI (before merger) and Nvidia would have been fabless if they had the long pockets of MS. MS could easily invest in a couple of fabs while putting only a small dent into their wallet.
That'd be such a money sink... for a company who's not going to be making that many chips in a given year. State of the art fabs are friggin expensive (they'd need one for it to be of much use for them). Just because MS has money, it doesn't mean they are inclined to spend it in places that won't get them the biggest ROI -- paying someone to fab their chips is likely still going to be far cheaper than building a fab, running it, and keeping it up to date. If their only use for a fab is for consoles and maybe a few other small things they are going to be wasting quite a bit -- they just don't have the volume need to justify such a thing.
Even if they are getting into chip design for the 720 or whatever they call it, I don't see any reason to believe they'd be, overall, better off (from their perspective) if they built a fab.