The Witcher (original, not the Enhanced Edition) shipped with a CD-Key that most people thought was useless. It allowed you to register your game with publisher Atari and get... not much.
However, the studio later released an Enhanced Edition, which added more cutscenes, more dialogue, more quests, two side-missions that stand alone from the main game, the official soundtrack, a CD of music inspired by the game and a "making-of" DVD. All this stuff was available for purchase; but the best part is the studio and Atari made all the new content available FOR FREE DOWNLOAD to all the owners of the original game who had registered their games using those previously mostly-useless CD keys.
The content could not be installed without keys. Of course pirates could just download cracked versions of the enhanced editions, but that's a humongous download, six gigs-plus and I doubt casual copiers would bother. Offering all that content free to confirmed, legitimate owners of the original edition wasn't just a nice thing to do, it was also a good incentive to have a legitimate copy of the game.
There were some problems in Canada - the bilingual manual was printed without keys. Oops. I'm one of the people who bought the original game and was stoked when I learned about the new content - only to flip through every page in the manual and find there was no fucking key. Good one, hope the proof-reader got fired for that. However, Atari support was pretty good, I filed a key request and two weeks later was happily slaying drowners with my silver sword - enhanced edition style.
Anyway, this might be a copy-protection scheme worth considering - downloadable content available only for legitimate, registered owners. I don't know how this would work with your game, but for me in my example, I thought it worked great (except for that shitty Polish download server they decided to use to release the enhanced edition content. Good idea, bad execution - make it EASY for customers to get the good stuff and they'll be less likely to visit TPB.)