There's no doubt about it, upon his return, he will definitely be a face. The only question is, will he remain a face, or will he turn heel shortly thereafter.
Everyone here has correctly stated that HHH is awesome in either role. If he is going to be a face, I hope he is not a joker-type, DX-type face; I hope he is more of an ass kicking, knock the shit out of everyone type of face. He's probably better as a heel, but I hope he returns as a face and stays that way for an extended while.
I thought I read somewhere, earlier in his injury, that he wanted to return as a face and finish his career as one. Who knows, maybe fatherhood has softened him somewhat. As well, with all of the blemishes on WWE right now with all of the steroid controversy and scrutiny, I feel they need a strong face character and a strong face champion to lead the company right now.
When HHH returns, the response he'll get from the crowd will be overwhelming. Remember when he returned from his injury last time? Did you hear the response his promos got on Raw this week?
Easy solution. HHH returns as a face. Becomes champion if not right away, certainly pretty quickly. Cena gets pissed off and turns heel again, where he belongs, and re-gains some of the fans that the sappy face Cena has lost. HHH feuds with Orton, Kennedy, heel Cena, Umaga, and lots of others. Plus there can always be face vs face battles as well with someone such as Lashley. WWE rides the wave until people invariably get tired of the "Triple H show" as people used to complain about before. Enter sledgehammer, and HHH goes heel again if they deem this to be his better direction. If not, he finishes his career as a face and surpasses Flair's "record" number of titles held.