NS2 will use Steamworks (Forum Post by Max). Steamworks provides services including Steam friends, server browser, matchmaking, cheat protection, auto-updates etc. 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 (Forum post by Max). 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 ( Newsblog). This was not the first pre-alpha-release.