Natural Selection 2 uses Steamworks. Steamworks provides services including Steam friends, server browser, matchmaking, cheat protection and automatic updates but requires the use of the Steam application.
Given that Steamworks is OS-dependent, post-release porting to other platforms will mean replacing Steamworks with a new system. However, Valve is making plans to port Steam to other platforms which would likely help this.
The first release on steam was in May 2010, But this was not the first pre-alpha-release.