Super Monday Night Combat is live and free to play on Steam (here) even though it was still planned to be in beta still. The launch came as a surprise to developer Uber Entertainment, but they decided to go with the release and kept it open for everyone to download.
Uber Entertainment's Ekanaut explains what happened: "We did a bunch of invites over the weekend and there was some wonkiness on how the messaging went out with lots of confusion which resulted in more silliness. We decided to just open it up. There will be an announcement about it on Steam soon." Wonkiness leads to confusion, confusion leads to silliness, silliness leads to a speedy Steam release.