The issue with Planescape is that it never was truly a different setting. The dichotomy does not exist. It was loads of detail on planes that are already part of the D&D multiverse, and then weird offbeat slang to go with it (not that there was anything wrong with the slang). Anyone who has the Manual of the Planes and Deities and Demigods is able to play a Planescape campaign. If you feel you're lacking enough detail in those sources, you can certainly go back to your 2nd edition materials. Between the Forgotten Realms, Races of Faerun, and Dragon Magazine, most, if not all the major PC races from Planescape have been covered. Savage Species tells you how to convert anything that hasn't already been covered.
As for Darksun, I believe there's an official fan site that's done some extensive 3E conversions.