Notch has been writing about why Minecraft hasn't appeared on Steam. I assumed it was because it wasn't finished, but apparently not. It's because of those pesky terms of service: "Being on Steam limits a lot of what we’re allowed to do with the game, and how we’re allowed to talk to our users. We (probably?) wouldn’t be able to, say, sell capes or have a map market place on minecraft.net that works with steam customers in a way that keeps Valve happy. It would effectively split the Minecraft community into two parts, where only some of the players can access all of the weird content we want to add to the game." (Were we expecting a vanity items shop in MC?)
Also, remind you of anything? Discussions with Valve are apparently ongoing.