I suspect any game funded this way would not be of triple-A quality. I mean, Obsidian is a pretty big studio and all. Most likely it would be a kind of side project, similar to the Double Fine XBLA games. It would also probably have to be smaller in scope than a game as story-heavy as PST.
I'd personally prefer seeing another Planescape game. Not a direct sequel to Torment; the story of The Nameless One is finished, and a sequel would just dilute the greatness of the first game. Unfortunately, the Planescape setting is, as far as I know, effectively discontinued, and I find it highly unlikely WotC will relaunch the entire line just for one game.
That, unfortunately, leaves me in a bit of a pickle. I'm not fond of Forgotten Realms, but on the other hand Mask of the Betrayer was absolutely fantastic story-wise (and not even written by Avellone, as far as I can tell!) so maybe there is some potential there.