I'd be willing to buy it about 2010 or so, it'd keep with the general trend of about 10 years between full editions of the game. That's the soonest I'd be willing to buy it, even if the material was an incredible improvement over 3.5 (which I can't really imagine, most of the tweaks I've seen are to setting-specific material and not to the rules themselves). I'm just not gonna shell out that much cash for a new set of core rules every 5 years or so.
I don't think everyone would buy it right away either. I know many groups that rejected 3.5 after playing it for a month or two and stuck with 3.0. I even know some folks that are still chugging along with their 1E and 2E products. Unless the edition police come door to door and burn all the 3.5 stuff, 3.5 games will still go on for several years after 4E comes out, no matter what.
I'm satisfied with 3.5 for the moment. Just because a new version comes out won't mean that I'd make a mess in my pants to get it. The core rules are just that- rules. Learning a new way of doing attacks and spellcasting and saving throws and all that needs to be worth the effort.
Reasons I'd buy 4E
1) The rules would have to be significantly different. If there was the same amount of change from 3.0 to 3.5 (very little), I'd skip it. I'd also probably skip it if there was a 4E SRD, and just use that to update what I wanted.
2) I'd want to see an entire random dungeon generation system (including random maps) in the DMG. There'd also have to be some pretty substantial page space devoted to designing the setting as well. Remember the 1E DMG? I'd want guidelines like that for filling out the blank spots on your dungeon and overland maps.
3) I'd want to see scalable magic items in the core rules, a la Weapons of Legacy.
4) I'd want the option presented in the Monster Manual to play at least common monsters (orcs, goblins, kobolds, et cetera) as a race, a la Savage Species.
5) I'd want to see a high-quality HARDCOVER Greyhawk sourcebook as the default setting. It'd have to be as high quality as the Eberron and Forgotten Realms sorucebooks. The generic fantasy world of the D&D core rulebooks doesn't need to be Greyhawk, but I'd want Greyhawk to be offered as the 'default' setting. That's one of the big reasons I was willing to look at 3E.
6) If psionics are going to be included in 4E, they should have the same rules in the core rulebooks as they do in the psionic books. None of this 'standard illithid' versus 'psionic illithid' stuff.
7) Raising the assumed levels. I'd personally like to see the core rulebooks provide advancement to 30th level. There are already several high CR critters (20+) in the core rules, and the proliferation of such creatures and the interest in epic level games suggests that players want to see the game shift up a few levels.
Reasons I wouldn't buy 4E
1) Eberron or Forgotten Realms was made the core setting. Eberron-specific elements in the core rules (such as warforged, airships, dragonmarks, et cetera) would also make me not buy it. Forgotten Realms-specific material (red wizard prestige class, Faerunian races or pantheon, et cetera) would make me strongly consider not buying 4E.
2) Character abilities were made more complicated. Most actions should be resolved by one or two rolls. I'm willing to sacrifice 'realism' to make the game play faster. I'd also want the rules and most creatures to be simple enough to run on the fly, if need be. We shouldn't be scrambling through the books when a grapple starts or a condition status is inflicted on the party.
3) It was inherently reliant on other products such as bundled software, miniatures, et cetera. I should be able to play the game with the core rulebooks, paper and pencils, and some dice if that's all I had.
4) I absolutely wouldn't buy it if it was any more like a video game. I don't want stuff like warlocks, nine swords characters, or reserve feats in the core rules set. Sorcerers are about as video-gamey as it needs to get.