It all depends. I'll do 4E if it's either a) close enough to 3E for familiarity, crunchiness, and playstyle (the good parts of 3E's way that is), yet different enough and improved sufficiently to be worthy of the investment, or b) a cool enough revision and upgrade of the degree that 3E was from 2E, yet retaining enough D&Disms (as 3E did) to still be familiar and worthwhile, rather than a total replacement that's only superficially like D&D.
I don't want a Castles & Crusades or True20 rip-off, and I don't want a GURPS, HERO, or Mutants & Masterminds rip-off. I don't want a D20 Modern rip-off either, even though it's WotC's own property. I'd also rather not see D&D become a miniatures game or computer game that just happens to be supported by roleplaying products and brand recognition. I don't know if I want miniatures to be as important to combat in D&D 4E as they are in 3.5E, but I certainly don't want them to become even more important or the entire focus of the game.
I could live with my 3.0 materials, my houserules, my homebrew settings, and the few 3.5 books I'm willing to buy (ToB:Bo9S, maybe a little else), and never buy another D&D product after this year. Not even a novel. 3E is crunchy enough and decent enough for me to tinker with and play/DM under for quite a long time.
Only exception: Since it's apparent that WotC has no love for old campaign settings except Forgotten Realms (GH only gets an illusion of loving attention), I would be willing to buy a later book (assuming the company eventually realizes the error of their ways) of Kara-Tur, Dark Sun, Planescape, Greyhawk, Al-Qadim, Spelljammer, or Birthright setting material and such, if not too horribly mangled in events/world changes, just for the sole purpose of cannibalizing for my 3E campaigns. And, well, to see if anyone else would be willing to run a 3E campaign set in one of those places, heh.
*note to self: begin devising plans for a thought-reprogramming device to eventually convert 3.5ers and 4ers to the rightful love of 3.0*
