Sending Jeff Hardy to ECW would take away from the value of the WWE Title he just dropped, and essentially erase ANY fan momentum the title run gave Jeff. So that's a non-option, really. WWE just put A LOT of money into marketing Jeff as a top, title-worthy star on SD, and moving him to ECW would kill all of that heat.
Moving Matt to Smackdown, on the other hand, creates an interesting dynamic that makes the show worth watching.
So where does that leave ECW in terms of faces? Well, the obvious choices have been mentioned - Evan Bourne (upon returning from injury), Finlay, and Boogeyman.
Outside of those three, a few people have to be out there to make the transition.
Rob van Dam. One time appearance, yes, I get it. But why not convince RVD to come back into the fold for one last run? He didn't high tail to TNA, he's made a few appearances to solidify the fact that he has a good working relationship with WWE - why not put him back on ECW and make Tommy Dreamer a heel GM, with RVD and Dreamer vying for who deserves to be synonymous with the ECW legacy?
Charlie Haas. Despite a hot gimmick, Seton Hall's own Charlie Haas has seen his in-ring and mic-time on Raw diminish, not due to a shortcoming on his part, but rather the fact that Raw is running a few very interesting storylines right now, and they just don't have the time to give him. Swagger vs Haas may be a decent feud.
Paul Birchill. Geez, how much time has WWE invested into this guy just to constantly keep him cold on the sidelines? He was strong in OVW, but since his call up, he's been one dropped push after another. He's got a hottie in his corner with Katie Lea, so why not give him a face run (lord knows she could suck face heat from a crowd like nothin') and a shot at the ECW title? It'd also serve to re-heat him for a future Raw run.