http://store.steampowered.com/app/400580
If you previously purchased the game from this website, you can use your license key to get a free Steam key by going here:
http://nettle.spikycaterpillar.com/bc.html
If you intend to run both a Steam and a non-Steam copy of the game, update your non-steam version to the latest available patch before installing the Steam build.
Trading cards are LIVE (and often worth the most on launch day) so if you're into that sort of thing, get in there.

Your CG and Achievement unlocks from pre-Steam do NOT carry over to the Steam version - you'll need to unlock them again. Which should give you something to do for those two hours to get the cards!