You can sign up for free over at the Club Nintendo site, and we warn you, while we signed up for an account without a hitch, when we actually tried to do anything (like register our games for those precious coins that will buy your rewards), the site got very slow, and even logged us out randomly more than once.
Apparently all of the good stuff runs 800 coins. If anyone has been able to register their games, please leave a comment as to how many coins you per each game. We've bought so much Nintendo crap that we're bound to have something free waiting...we'd hope. [Club Nintendo via Kotaku]