Some day be Dead Space 3 on Steam?

Never.

Yes, of course. When Valve buys EA.

Soon.

Nope... Then Mass Effect 3, Battlefield 3 and NFSMW 2012 should be on Steam as well :P

Same as Crysis 2. If they release a version that makes it possible to let people buy all the DLC through steam (too) or a version that already contains all DLC like Crysis 2 maximum edition, then we can expect a steam release. Though I've read Dead Space 3 has microtransactions...those probably don't go through steam channels so it won't be allowed on steam.