Depends. If they make Planescape, or Dark Sun, or Spelljammer or even Ravenloft, staying faithful to the style of the original and not developing it well, I may pick them up and mine them for ideas.
Looking at the fate of the Forgotten Realms, though, I don't have high hopes.
I'd buy most of them but mostly for nostalgic reasons and not so much because I want to use them as a setting for my game.
I prefer homebrews that borrow elements from various settings and other sources.
Eberron and - depending on what they're doing with the setting - Dark Sun are the only ones I'd consider using 'as-is'.
I'd probably buy Planescape and Ravenloft because I enjoy reading stuff from these settings.
I don't care enough about the other settings to consider buying them, unless they get rave reviews.
I won't buy anything related to the Forgotten Realms because of a strong personal dislike for the setting.
I also voted for 'other' since I would really love to see a completely new setting as well.
I voted for Eberron, Dark Sun, Planescape, and Ravenloft.
Eberron is my favourite Third Edition setting, and one of my two favourite D&D settings ever. Planescape is the other. Ravenloft is always a lot of fun. Dark Sun is more appealing to me now that I can use Fourth Edition for it than it ever was with the clunky adaptation of Second Edition.
These are the settings for which I would purchase the Campaign Guide. I would have an open mind about picking up the Player's Guide for other settings - Forgotten Realms, for example - simply for the character options and whatnot. I'm also considering the Forgotten Realms Campaign Guide, but I'm not sure yet; need more previews.
I'll be staying primarily with a homebrewed setting. However, depending on the updates to the settings, I could be convinced to grab the Realms, Eberron, Greyhawk, and Spelljammer. I'll also be grabbing Pathfinder's setting as well.
Of course, an update would have to be more than a thinly-veiled retake on all the information in the dozens of other books on those worlds before I'd be interested.
So long as they stick to the 3 book (really 2 plus an adventure) scheme, I'll likely get them all. I'm a collector. I'm fine if a really good extra supplement comes out for a setting once in a blue moon, but it was my fear of constant supplement releases for settings that drove me to not buy the books for any in previous editions. I love the idea of being able to pick up two books and "having" the settings.