Last time I checked you could do a Steam release in addition to other distribution channels, but it indeed can be a problem because Valve may put some constraints there. Notch wrote something about it
here, though it's a bit odd and unclear, perhaps intentionally.
I see Steam mostly as a marketing platform that can bring a lot of players that we would otherwise miss. However, I can bet there will be some complications, because of how we manage our users and store data in our backend cloud etc.
O NO!!! Your going to put this on Steam?
The question is, what do you mean by
this?
This is an engine, and you can't put an engine on Steam. If we are going to put something on Steam, it would be a single game. And probably it will be worth the effort only if we can make the game a gateway to the Outerra World. Honestly, given that we aim for the OW to be a platform, that makes it somewhat a competition to Steam ...