Hmm. The simple answer is - I agree with TheDevilshark.
The more complicated answer is:
Probably, in some form rather unlike the usual Steam hosting/sourcing-for-a-percentage model.
CIG has / will have relatively huge infrastructure to host the 'Verse. THEIR server infrastructure will be integral to playing the game and interacting with the 'Verse. It also would NOT be in their interest to have to create a Steam-compliant duplicate of the game for distribution via steam, and to have to make and distribute any patches / updates through that channel as well as direct.
That being said, Valve/Steam has huge sales power that they probably would be glad to work out a deal to exploit. And SC should have a huge potential audience that Valve would like to have cut of.
So I can imagine some future deal where a Steam-offered Starter package or key was available via Steam. Then, upon install, the reliance on Steam is severed and your game is connected to the CIG infrastructure is just as if you bought from their website in the first place.
Does Steam market WoW or other MMORPGs? Because the necessary technical connections would be a similar set of problems.