An older post, but since it's been going on for a while, I may as well weigh in on this one. If I repeat information, then that's because I didn't feel like reading 15 pages of waaaargablblblble about psionics. I hear enough of that with my group :)
Psionics is the second thing done with D&D. Literally, the fourth book released for the various editions of D&D are usually psionic. Look up Eldritch Wizardry sometime; it was fourth "collector's edition" D&D books printed back in the early 70's, and detailed stuff like Demons, Gods, and a strange power called psionics. The fourth 3rd Edition book was the much-cursed Psionics Handbook, and the fourth 3.5 book was the (slightly) better received Expanded Psionics Handbook.
I don't use it, myself. I've used the Vancian system for so long, it's practically how I think. A friend of mine, though, swears by psionics. He's been playing psionically gifted characters since 1st Edition, where the rule was that during character creation, you had to roll a 95 on a D% to be psionic. That was almost on par with making a paladin or a bard back then.
But psionics were always there. It doesn't feel right if there isn't a massively broke form of magic looming over the horizon, just waiting to spring on your campaign. I've been lucky, since only ONE friend is a psionics fan, and everyone else avoids it like the plague. But I've seen the downside to psionics, which is the same problem with vancian magic only more so: the lure to blow all your power points for three rounds of god-like power is huge. So huge, that I've often seen my psion friend giving everyone the finger because they're telling him to pull more horrifically powerful abilities out of thin air, but he's out of points.
I'd almost be tempted to say that psionics don't need much in the way of conversion. Most of the way that the casters work in Pathfinder are reminiscent of psions and wilders. Especially wilders. The only real conversion required would be something along the lines of the at-will cantrips. Detect Psionics at will doesn't seem too bad, for example.