Microsoft owns the brand, and the brand is what counts, the brand is what sells the hell out of the game...
Now, Bungie could do a game for the PS3, but Sony doesn't need Bungie, i respect them as a company, but they have nothing against devs like Insomniac or Naughty Dog, and i'm only talking about devs that work exclusively with Sony, if i throw 3rd pary the list is way bigger... So for game quality, they don't need Bungie... Now, they could do a game and advertise it with "from the creators of the Halo series" to build some hype and get buzz, but it has to be a game worthy of the hype...