It will tell you it needs to activate on Steam as soon as you press install, it opens Steam client, you enter the code you get in the back of the manual to activate, then Steam immediately shuts down.
So yeh, you can't do anything at this point.
Basically, gotta wait till whenever Steam activate which is likely to be US time so 2 days from now. So I have no idea whether Steam needs to be on or not after that.
Seriously I dunno why some people are so apologetic for online activation. The street date for the game in Australia is tomorrow which is the 18th. Going by that, that would mean the game is effectively useless till the day after it's release anyway. Some stores broke street date which is when I got a call to pick up my copy. I have it, it's legit. The fact I can't play a legitimately purchased game is in my mind ludicrous. I hope the same names telling me to fuck off right now are swinging the same routine in the next 'DRM is shit' thread.