As an electronic publisher, I'll be supporting 3.5e (and by extension, 95% or more compatible systems such as 3.0, d20 Modern, Modern20, several of the Mongoose OGL lines, Grim Tales and Star Wars Saga Edition).
I'm currently trying to determine if there's sufficient market to support FATE or RuneQuest (and if FATE steps too much on the toes of my proprietary system for me to want to release both). Once I have products out under as many straight open licenses as possible, I'll also look into licenses that, upon cursory inspection, seem to have a bit more complicated setup: Spycraft and True20, for example.
I'll also begin supporting 4e as soon as I can get the SRD - whether that's before the core books release, concurrent with their release, or any given amount of time afterwards. Since I'll have *lots* of material needing nothing but a rules conversion, I hope to be able to put out one or more short products within a week or two of the SRD's release, and certainly no more than a month - it really depends on how familiar and easy to grasp the system is, how long it takes me to learn its broad strokes.
As soon as any of those ceases to be a viable market (by which I mean even the relatively small amount of time it takes me to convert the rules and do whatever small layout changes are needed ceases to be worthwhile), I'll stop supporting it. I'm not really sure that will EVER happen with the current build of the d20 system; certainly I don't see it happening until d20 Modern gets a WotC upgrade, if it ever does.
If another company, be it Paizo or Green Ronin or Mongoose, were to release an open system with the explicit intention of serving as a D&D replacement, which is what a "3.75e" would be, I would support it as long as it seemed like a viable market, too.
But I fully expect the vast majority of my OGL sales to come from 4e. If I were a print publisher, I'd be VERY reluctant to produce anything else (in the d20 market, anyway) once 4e comes out, because I wouldn't have the benefits of the long tail and near-free storage and distribution.