Cam Banks
Adventurer
Outside of the two settings I know 4e is going to support (Eberron and FR) I voted for Greyhawk, Mystara, and Dragonlance. Greyhawk should get some major loving this time around without being any kind of implied setting; Mystara is perfect for 4e, since I think BXCM is the closest version of D&D to 4e in terms of the feeling I get from it; and Dragonlance because I'm already planning an unofficial 4e conversion for the Dragonlance Nexus and if WotC wants any official DL design work, all they need to do is ask me. 
Cheers,
Cam

Cheers,
Cam