Pale: Good idea, BUT it *can* be abused. Case in point: When Nintendo released the Metroid Prime 2 demo via having a certain number of games registered, people started trading codes online. How? Simple. Go to Blockbuster/Hastings/Hollywood/etc and find the game section. Most of these places leave the registration information in the box, so someone could theoretically register all of those games under their name. Hell, they could make up a few aliases with different e-mail addresses if they wanted and double/triple their number.
Granted, that doesn't take into account having, say, your DS reg #, but there are ways around that too. For example, my nephew didn't register his and he never will. That's another one for me. Plus if I ever bought one as a gift for someone who never used it, I can take their code. Or you can just look at the number through the hole in the box (if Nintendo still does that, I dunno if they do).
Then you aren't far away from having lots of new DSes headed your way.
Nintendo could block it by 1) forcing people to send in the broken DS first, or 2) making the # of registrations needed very high.
In any case, it *could* be exploited with a little work. That's all I'm getting at. Personally I think it is a great idea.
Somewhat offtopic, someone once on Slashdot suggested that, in order to help publishers' plight against used game sales, people could send a game they didn't want anymore to the publisher, who would then send them some sort of coupon/voucher toward future purposes. Then the company could repackage it and get the profit themselves. Not only that, but they could sell them directly online, and could even have standards for including the manual/inserts. Think about it - no more finding a disc only copy of Ikaruga. Instead, you log into Nintendo.com and buy it yourself because someone got tired of it. That way Nintendo makes a small profit, the original buyer gets a coupon and generates more profit for Nintendo, and you get a killer game AND Nintendo gets more profit. Win-win-win all around.