Re: Underworld -- supposedly Gods & Heroes (set in ancient Rome) takes you to Hades -- so there you go.
Re: WotC -- I know very little about WotC/Hasbro but I think all Hasbro IP is handled by a specific group that is not WotC. I'm sure if money was on the table a deal would happen whether or not Planescape is an active setting or not.
Re: Topic -- I loved PS:T (and am actually replaying it right now) and think a Planescape based MMO would be amazing if done right and would totally suck if done poorly -- just like every other game ever made. I really can't fathom the negativity by posters on this thread to a hypothetical game.
Planescape remains my favorite D&D setting and it'd be great to see any type of CRPG (solo, multiplayer or massively-multiplayer) set there.
Indeed if the upcoming Planescape mods get good reviews I'll buy NWN2
