CM Punk retaining the belt seems obvious to me.
Big Show was thrown in there just so they can save the big Punk-Cena one-on-one rematch for the next month and so they could add a little something 'extra' to the match. Kinda like when Show was thrown into the triple threat at WM25 (just so it wouldn't just be Cena-Edge again) or when Kane was thrown into the Bryan-Punk feud a couple of months ago. For that reason, I'm fairly certain that Big Show is eating the pinfall. It may even be both Cena and Punk who pin him together.