I don't think Greyhawk WAS the default or implied setting of 3E. Other than the gods, there was never any real mention of Greyhawk names or the like.
The Realms strikes me as a much better vanilla setting, especially given its connection to the computer games (Demon Stone, Neverwinter, Baldur's Gate, etc.) that makes it more familiar to newbies.
I hope we see new, cool settings for 4e, not a rehash of older, failed settings (although I do like Planescape, I can see how it wouldn't appeal to nearly enough people to justify making a CS).