Everybody needs to throw some ice on their "Cena's gonna go heel" hard-ons. It's not happening.
As a babyface - and as the face of the company - Cena is pure gold. Not only does he connect with the fans better than we've seen anyone do since Austin and The Rock, but he has a squeaky clean track record, he works his ass off in the media, and he's a company guy. He gets Make-a-Wish foundation requests similar to Hogan in the 80's. He's a huge part of the WWE's efforts to support US troops. He sells merch like nobody since The Rattlesnake.
Wrestling fans got spoiled by the Hogan heel turn. It was such a shock moment, because until then, nobody thought it possible. But remember - the REASON that happened was because Hogan had jumped ship from the #1 promotion to the #2 promotion, and the momentum created by his arrival had started to wane. As Eric Bischoff said in his book AND in the WCW DVD, he needed to make a loud noise to get people's attention. Hogan was that noise.
WWE tried to replicate the Hogan heel turn by aligning Steve Austin with Mr. McMahon at Wrestlemania 17 and into the 2-man powertrip. Austin was tremendous (he worked the 9/11 angle with Kurt Angle as the heel and was brilliant, IMO) but the fans would NOT let go of the nostalgia. Plus, since those fans had already need through the Hogan heel turn, we were wise to it. "Oh, I get it, you want us to hate him now. Well, I don't wanna. So there."
If Cena ever DOES go heel, you're looking at a MINIMUM of 2 years down the line, if not longer.
Furthermore, if there is any "grooming" going on, they are grooming Orton to be the top face on one of the two shows, with Cena on the other. Personally, I'd love to see Cena be the top face on Smackdown because he connects better with the younger audience, whereas Orton can be edgier and connects with the older Monday night crowd.