If 4E is to 3E what 3E was to 2E (ie, a massive improvement in rules clarity, ease, flexibility, on and on), then I'd like it right now.
I have no problem with moving to a new edition. I buy few D&D books, mostly just the core stuff and a supplement here and there. I wouldn't have a massive collection of outdated books to deal with. Just a new, sleak system I can start playing and enjoying right away. For $20-25 a book (amazon prices, where I almost always shop for D&D stuff), I'm not out much money to get started.
The only thing that would tick me off about a new edition is if they changed the mini scale again and/or made all the existing plastic D&D minis obsolete and unusable in the battle system. Hopefully with the success of the D&D mini line, they will only tweak it in minor ways leaving all of those existing products ready to drop into any 4E game (sans stat cards which would need updates for the rules I'm sure)
But to be a little more realistic, I'd like it as early in 2008 as possible. If they announce it in Feb of 2007 like some are speculating, then I'd like them to release it in Feb of 2008