See, I kinda think Castiel taking him out of hell broke the deal. I mean, pulled him out to do God's work? I seriously doubt God would let him go back to Hell when he does (eventually) die.

I mean, papa Winchester made a deal, and he escaped from Hell when the boys opened the Devil's Gate at the end of Season 2. He didn't go back to Hell. In the latest issue of Supernatural magazine, Sera Gamble says John "went on to a better place". (I'm paraphrasing, but still!) John got out and his deal was still in effect, basically.

So I would think the same thing applies to Dean. He got out, they can't take him back. If they could, surely the Hellhounds would have already tried?