Mr.Spatula: They could make a new game in the same setting (though I believe planescape was never updated for 3rd edition) with a totally new hero and story (and maybe setting), but what would be the point of slapping the torment name on it except as a lame attempt to sell more copies based on the first one's following (similar to what happened with far cry 2)?
Maybe not in 3rd Ed, but definitely in 4th ed - its in the Manual of the Planes. Also, considering the title was separated by a colon, it would definitely not be "Torment 2" but I'd imagine it would be a new protagonist with new companions (and maybe the occasional cameo of original companions) set in the same 'verse. You could essentially have a near-limitless franchise of "Planescape: XXXX" titles.
Feargus, however, is right. There is such a love of PS:T that any re-imagining or re-boot will likely draw tons of fanboy ire, caffeine-induced internet rage, and numerous angst-ridden diatribes equating Feargus to George Lucas and 'new character X' to JarJar Binks. For a glimpse of what I mean, look at comparisons between the original Fallouts and Fallout 3/Fallout: New Vegas or (my personal favorite) the Star Trek reboot a few years ago that had people all up in arms and hyperventilating about 'breakign cannon' and 'J.J. Abrams hates Trekies' and all that horsecrap.
That being said, I'd love to explore Sigil and the planes all over again. Much more fun that the run-of-the-mill elves/dwarves/evil necromancer rehash which flows from the Fantasy RPG nozzle anymore.