Nice to see (some) people finally breaking away from the consensus that next year's setting will be Dark Sun. Athas is a great setting that deserves to make its 4e debut, but it just doesn't fit the clues we've been given so far from the designers that the new setting:
a) will commemorate an anniversary
b) was very popular in its day
c) will tug at the heart-strings of longtime gamers
Athas was never that popular with the broader gaming community and doesn't have a significant anniversary coming up, so unless one thinks WotC has become a serial fibber regarding the 2010 setting, we need to look elsewhere. Also remember that WotC recently saw fit to can the designer most responsible for the current Dark Sun setting, which further reinforces the idea that Athas will breathe for a year or two longer before making the jump to 4e.
Dragonlance, though, fits the available clues quite well...