I obviously don't know what WWE is planning for him, but if I were in control, I'd put Harris on Smackdown and team him up with Chuck Palumbo. A few reasons...
1. Harris isn't big enough to deserve a push right off the bat. We aren't dealing with WWE signing someone like Sting or how they're bringing Big Show back who was previously in the main event scene.
2. Palumbo is clearly acting like dead weight on Smackdown. His feud with Jamie Noble is being held up just by Noble (and at times Michelle, which is pathetic), and he isn't receiving enough heat one way or another (face or heel) to really go for a midcard push. If you give him a tag team partner, however, then you help the tag team division, which is really lacking in the WWE right now.
3. Raw is going to be fixed in due time because of the draft. If Harris were to go over to Raw, he'd be in the midcard, but I just have a really good feeling that Elijah Burke is going to be drafted to chase the IC title instead.
4. ECW is a possibility, but are they really going to try to push him towards the main event scene on their C-show while they're trying to build up Shelton as well? Doubt it.