I'll be honest, I haven't actually tried this yet, but in my other GFWL games, you can play without being logged on to GFWL, you just won't get any of the acheivments or be able to use your bonus wargear (like the commander wargear set or any pre-order wargear you got).
The thing is, since you have to be logged in to steam to play this game, you need to be connected to the internet anyway, so you might as well log in to GFWL.
Edit - Okay, jsut checked this, you can't do anything without being logged on to GFWL, that kinda sucks, can't even use the army painter.
Unless you want to mess around with the game files, I don't think there is a way around this, however, as I already said, you need to be conencted to the internet anyway for steam to work, and you can get a GFWL account for free, so there shouldn't be too much of a problem.
[Edited by arwn, 3/7/2009 7:06:43 AM]