The update will bring over 50 Steamworks Achievements, along with "a host of new features and enhancements in AI, Server Browsing, Demo Recording, Mod Support, and more." A more detailed list of the upcoming changes is available here. Steam registration is only required for the achievements. nope Registration is a simple matter of clicking the main Steam menu option "Games" and selecting "Activate a product on Steam."
Any CD key should be eligible for activation, including versions purchased from other digital distribution services. Also note that the activation will require a new installation of the game, so users will want to delete the old one in order to avoid pesky hard drive bloat. [Thanks Visceral Monkey.]