A post on the Space Marine Forums from Sunday night outlines how co-op support will be added to Warhammer 40,000: Space Marine in a post-release update to Relic's action/RPG (thanks Shacknews). It's nothing that will affect PC gamers, but this will be one of those disincentives to buying used copies:

Co-Op Details:

1.) Space Marine will have Co-Op added in 30 days give or take after launch.
a.) Why? We wanted to polish up the Co-Op aspect of Space Marine.
2.) The GameStop Co-Op call out was old information and we have a nice slate of info to cover all your Co-Op needs.
3.) Everyone who purchases Space Marine at retail (brand new) will get Co-Op for FREE. If you buy the game one year from now the Co-Op option will still be Free as long as the game is sealed brand new from retail.

I will update this thread with all the Co-Op details when we release them, so please stop speculating and confusing users with false info.