Or am I restricted to second hand?
Edit: As mentioned by LessPop_MoreFizz, as of today (9/28/10) you can get Planescape: Torment from GoG. Buy it now!
OLD ANSWER: It's not available digitally anywhere. As Daemonic mentioned, keep an eye on Gog.com. It'll probably show up there before anywhere else.
In the meantime, it's VERY findable on Ebay.
As of today (9/28/2010), Planescape Torment is availiable from Good Ol' Games, for the low low price of $9.99, complete with some fun new bonus material.