I'm one of the people who does not want to move to 4e. And that isn't just because I don't like the Spellplague thing that got done to Forgotten Realms. I also don't have time to relearn all the D&D rules.
But I'm thinking of giving Pathfinder a go. That is fairly similar to 3.5, but there are some important changes and I think that it would need a "3.75 PSCS" update to deal with them.
In case you have not noticed yet, Pazio have put up a Pathfinder SRD (called the Pathfinder Roleplaying Game Reference Document).
This is released under the OGL, so everyone could use that to help them make a Pathfinder conversion.
There is a fan made version that is more like the Hypertext d20 SRD. That might be easier for people to use. It is called d20PFSRD.
So if there are any Planescape fans here who are interested in Pathfinder, I think it would be easy to skim through the core rules and not have to worry about pulling out Paizo's PI.
I like Pathfinder and have written some planar stuff for it here and there on this and Paizo's boards.
Much of the system is basically 3.5e, so not too much change is required mechanically. Pathfinder's core classes are out of beta and are now different, which would necessitate some changes to stats for NPCs, but little else along those lines. I'm not even sure it needs a formal conversion.
When it comes to the world, of course, Golarion's cosmology is substantially different from the Great Wheel, with its own arrangement of planes and its own pantheon. Put me down as one who really likes the arrangement of the elemental planes. I bought The Great Beyond and enjoyed it a lot, and recommend it for planar gaming in Pathfinder. (I have a review elsewhere on the site. I bit down hard on one particular point that jarred me, but I can live with it especially given later comments from the authors.) Beyond the Vault of Souls is also a pretty cool campaign, and I look forward to more planar stuff from Paizo.
Todd Stewart, a.k.a. Shemeska the Marauder on these boards, suggests that a city briefly referred to in The Great Beyond called "Galisemni, the city of the celestial and the damned," would make a good Sigil-substitute for Outer Sphere gaming. So could Shadow Absalom, or you could simply put your Sigil-clone floating in the Maelstrom, Lady of Pain and all. The Outer Sphere is basically the Outer Planes, really, except for the eight lesser interstitials between the major alignment planes. If you want them back, I'd just make Carceri a layer of the Abyss, and Pandemonium an evil-tainted region of the Maelstrom. Ysgard is basically Gorum's realm and the Beastlands could be found in Nirvana. Bytopia would probably be a goodtinted region of the Maelstrom, I'd think... or a peculiar realm in Nirvana. Arcadia would be a pastoral realm in Heaven, Acheron one of the borders of conflict between Axis and the Maelstrom, and Gehenna a region in Abaddon.