Greyhawk I'm completely neutral and unbiased about as a new setting. It is all a practical reaction. If they can do a good 4E version of it, go for it. Otherwise, why bother?
With Dragonlance, I have major reservations. Now, I don't know how much of my reservations are based on liking the setting more for novels than gaming. But it seems to me that the intersection of people who "like Dragonlance a lot" with those who "like 4E a lot" is awfully tiny, compared to a lot of other things that you can do. My practical side can't believe that there is enough of a market to warrant such a product--even if you can find a way to do a good 4E version of it. Now, maybe a new slant on it, sufficiently inspired, will bring in a bunch of new fans that aren't wedded to what went before. But some of the Dragonlance gamers are already really prickly about canon.
So I'd prefer a new setting specifically geared to 4E over either of the above, as I think it practically has a better shot of being worthwhile.
But if they absolutely must dust off an old setting to do a 4E version, I vote Al-Qadim. Nothing has been done with it since the 2E days, and those 2E setting books are expensive. And like Dark Sun, there are some elements that might be done more naturally with 4E than in the original. I'm not positive how much, because I got into Al-Qadim late the first go around and missed a lot of it. But I certainly remember the tone and setting being something that I can see 4E doing well--at least the Arabian Nights and Sinbad inspired bits.