Of course I'd be excited. It would give us something different from what we've seen for the past however many years Cena's been face.
As Wade says though, the question is do WWE have enough depth on the face side of things to cover loosing Cena? The answer is probably no. Punk and Cena are the top 2 guys in WWE, and both would be heel if Cena were to turn. There's also talk of an Orton heel turn coming, that's 3 of the top guys heel.
I think that Punk would probably have to turn face to accommodate a Cena heel turn, and even then, they're still short IMO.