Well, I don't think the people within Bungie have the deep enough pockets to pay full price for a majority share to get their studio back...they're worth alot more now.
But they've never really slagged on making Halo, they love doing that, just not only that...which they are since they're working on at least two games now (one of which is a Halo game). From all I've read and heard on the podcasts it seems the independence had a lot to do with the atmosphere in the studio, being in charge of your own fate and of course getting a bigger cut of a big hit. Now while MS payed only 40million $ for Bungie, now they're much more worth than that and I believe some people within Bungie bought back a majority. MS still has a minority % of Bungie, I think, and they made a deal with MS in order to get 'their studio' back but of course under heavy conditions, you just don't get back independence magically, for free, even if you're Bungie.
Bungie/Frankie made it pretty clear they'd be making games with MS and for 360 for a long time. What changed is that there's even less oversight from MS and they pay for working cost of running the studio themselves and they feel more ownership of their studio.