If you've played RPGs, you'll absolutely know what Planescape: Torment is and why you should be excited to know that the game has just appeared on the Steamworks backend as an upcoming title.
Should Planescape: Torment materialize as a downloadable and purchasable title on Steam, it will be the first Infinity Engine game to see release on Valve's digital distribution service. It's unknown whether its release would serve as a herald for seeing Baldur's Gate II or the Icewind Dale games on the service, as Atari (and now Beamdog) hold the rights to at least some of the other games.
The first Baldur's Gate and its expansion pack have since been released in an Enhanced Edition this past week, so if you're aching for some Infinity Engine action, I'd recommend just playing that until something more becomes of Planescape: Torment.
Alternately, you could always grab Planescape: Torment and all the other games from GOG.com, where they've been available for over a year now. Is having those games on Steam really that big a deal?