My homebrew (designed for 2nd edition) could probably convert fairly easily, but I'm not even using it for 3.5 these days - we're doing War of the Burning Sky in what in theory is Eberron, and the plan is to move onto Pathfinder (using Paizo's world) after that.
Strangely, the advent of 4th edition has made me want to do a new 3.5 homebrew, now that I know WotC aren't going to be producing any more crunch for me to consider. I've been looking over my ton of 3.5 stuff and picking the bits I enjoy.
The result is pretty weird - classes allowed are warblades, specialist wizards, scouts, psychic warriors, warmages and favoured souls (with paladin, ranger and bard available as prestige classes).
I'm not sure whether I'll ever pick up 4th edition, but if I do (or even if I get to reading the SRD) I'll probably both convert my homebrew and design a new setting for it.
Like most of the people here, I like designing worlds
