It's already Steam greenlit. Just not released quite yet. They're waiting until they have all the new features added and working in a completely stable build before they do that. I bought initially through Desura, which is very similar to Steam, but is almost entirely, if not entirely, indie games.
It's the first game I've ever bought that was still under development. The experience has been pretty great. The devs are really active in the forum, so as a player you can actually have an impact on the direction of the game. I've seen people's ideas from the forum get picked up and implemented several times now. It's really cool. I've chatted with a couple of the devs personally a few times now, and voiced some of my own. Not an experience I've ever had before with a game. So much as one that I like so much. It's not even officially done yet and I'm already completely addicted.