AFAIK, whenever a major company buys somebody up, they make the top people sign non-competition contracts, exactly to prevent high profile leaders like the Stampers from suckering people like MS into buying the hollow shells of their companies, jumping ship, and then recreating the "real" companies somewhere else.
The Stampers might gather ex-Rare people around them because they might all know and like each other, but if the Stampers have non-compete contracts, it would mean "the new Rare" won't be making any Nintendo or Sony games. They might be able to make XBox games if MS deigns to let them.
If they wanted to do the same sort of thing that they did before (making videogames), it has to be in some distant field, like cell phones or Leapfrog devices. And even then, if they scrape out some moderate success, MS can just move in on them and make them pack up and leave at just about any time. Of course, that's if the Stampers made that sort of contract with MS (I wouldn't know). But if they did, for all intents and purposes, they're gone and they're never coming back.
Of course, Jordan Weisman/FASA is sort of back, so there's room to hope.