While I can't speak for Rebellion, but given DCs recent stance on unsolicited submissions (they won't look at them any more) it's likely most companies will follow suit. (DC landed in trouble because they were sent an unsolicited idea which wasn't used, then, a few years later the idea turned up in a graphic novel. I'm sure this IS coincidence -- they idea was fairly generic - What if Superman was rocketed to Krypton from Earth! - so it's possible that more than one person would've come up with this.)
Most movie companies will only look at unsolicited submissions via an agent and since agents are sharks (frequently conning poor dumb sods into thinking they're literally minutes away from selling their script) I wouldn't even bother trying. Work begits work. You generally need to have some sort of reputation (preferably a good one) before most companies will even entertain the notion of your scripts.
-mr c
