Eh, there's always the old steel cage finish where the powerhouse face nails the heel so hard, the latter goes flying and breaks through the cage door, being declared the winner because he escaped first. I know HIAC is supposed to be different, but it's not like promoters don't "re-invent" rules when it suits their needs. Look at how many times--including last night with Gabriel--someone wins a match clean and they "forget" to mention it's non-title, just to cite on example.
And, yes, this means Foley taking the insane roof bump and going to the ground should have been declared winner in that match, but like I said, rules can magically change. They could even have some sort of promo on Raw the next night saying how Punk outsmarted Ryback because he had the first-out stipulation written into his contract, never mind that Vince personally presented the contracts on Raw.
Just can't see a company as cautious as WWE giving the big strap to someone who can't even promo, so I expect Punk to retain...if they don't pull a substitution, as some have suggested above...which is bound to get them some disgruntled PPV buyers who are going to feel like they got a bait-and-switch since Ryback vs Punk may be why they ordered in the first place.