I would guess Hasbro to start.

There's an easy way to find out. Make a website with a download link for a cracked version of the game, advertise it on Twitter, Facebook and YouTube, and see where the lawsuit comes from :-D

I'd guess Interplay owns the rights to Planescape. Black Isle Studios became (sort of) Obsidian Entertainment, but PS:T was published by Interplay.
Unfortunately, Interplay is in enough trouble with the Fallout fallout... <you see what I did there?>

I own the rights and I refuse to sell 'em!

Atari acquired the rights to all of Interplay's other D&D titles when it licensed D&D itself. But it doesn't seem like Planescape was a part of that.
So it's probably in rights limbo between the two. Either Interplay were charging more than Atari were willing to pay or WotC simply don't want Planescape licensed titles sold any more.
Or something else entirely...

Okay, as you want. Don't move. Imma just gonna grab me boomstick 'nd be right on me way.

I thought we had shared ownership and I am willing to sell my Torment half! :oP

PlaneScape was on GameTap a while back, so someone must know who owns the rights.

Hasbro/WotC owns the Planescape IP tho not sure about Planescape:Torment
Last I heard Interplay never sold the rights tho no idea what goes on behind the scenes...

Yep, it's quite a complicated issue. Do the even companies themselves know it for sure? Perhaps its too big of a beehive to start messing with. Anyway, Planescape: Torment is one of my all time favorites, so I hope that we will someday find out its owner and get it in GoG.

Let's see....
PST ran on a (modified) version of Bioware's Infinity Engine.
Interplay published this Black Isle masterpiece, long ago.
Interplay still exists...for now....
Wizards of the Coast and Hasbro own the D&D Franchise.
Atari has the rights to actually make D&D licensed computer games.
Well, looks like there's at least a bunch of jumps of hoops to get through before anyone can re-publish this thing.

Didn't they also 'kill' the Planescape setting?

Planescape material for 3.0E and 3.5E [url=http://www.amazon.com/Handbook-Dungeon-Dragons-Fantasy-Roleplaying/dp/0786934298/ref=cm_lmf_tit_3]been (at least I *think* those represent Planescape...), and I have no idea what's coming for 4E, other than what [url=http://en.wikipedia.org/wiki/Dungeons_%26_Dragons_campaign_settings]Wikipedia says:
Campaign settings in the 4th edition of Dungeons & Dragons consist of only three books each: A campaign guide for the Dungeon Master, a player's guide for players and DMs, and a ready-to-play adventure. There will only be these three books for each campaign setting, although a new setting will be released each year, and all settings will receive continuing support in D&D Insider. As of 2009, only Forgotten Realms and Eberron have been released, with the 2010 setting to be Dark Sun.