Dragonlance, Dark Sun, Greyhawk and Ravenloft were all popular in their day, and all have vocal followings, so the anniversary clue would be key. 2010 is the 25th Anniversary of the Dragonlance novels, and the 20th anniversary of the Ravenloft campaign setting. Dark Sun, meanwhile, celebrates its 20th Anniversary in 2011.
If they want to leverage anniversaries, I'd say it'll be DL in 2010, and DS in 2011.
This seems likely to me. DL is popular enough to be one of the next setting books to be released. The only problem is that the setting was very much tied to the 1e rules originally. This wasn't a huge problem in 2e, and I have no idea how it worked in 3e. With 4e I'm not sure because of the drop of Vancian magic, which was pretty heavily tied in with the Orders of High Sorcery. They could still work it out.
I'd imagine Dark Sun is probably under consideration. It has a somewhat strong fan base, and it's usually counted among the good 2e settings. The only problem is trying to shoehorn core 4e classes and races into the DS setting; they tried that with the 3.5 stuff in Dragon, and it didn't go over well at all. If they're just doing limited support for setting with a player's guide, a DM guide and a module, and just continuing support through DDI, there shouldn't be any reason for them to not cut some of the core stuff out. It's better to present that stuff as options than say, "You have to use everything from this book if you want to use anything".
Greyhawk is harder to tell. It's the most classic setting, but a lot of the fans haven't always been happy with new material, and that goes back to 2e. If the people who are most interested in Greyhawk aren't interested in playing it with the 4e rules, then it may not be worth it for WotC to try to revive it again. Then there's also how much in the world needs to be adapted to accommodate some of 4e's changes which also will not sit well with some of the older fans. Doing Greyhawk with be a Catch-22 for WotC no matter how they approach it, I feel.
Birthright's an iffy case. From what Perkin said, WotC may not fully revise the entire setting, but use the rules for the domains and stuff in a wat that any DM can apply it to any setting. IMO, this is really what TSR should have done with Birthright back in 1995 instead of creating yet another setting to clog up 2e, and if an example setting was needed, they could have grafted it onto the Realms or something. But, OTOH Birthright's setting had some really good ideas, it was at least more interesting than a vanilla kitchen sink setting.
Mystara I think is unlikely. As Perkins said, there's no hook. There's really nothing in Mystara that distinguishes it from Greyhawk or the Realms, and D&D doesn't need three generic setting that cover nearly the exact same ground. Yes, it's home to the B/X modules, the Savage Coast and Hollow World, and there's a good deal of stuff there upon which DM can build a campaign, but then again, Greyhawk and the Realms do just about the same thing. As a whole, the three settings differ only in the details.