This is a common problem with Lites. Amazingly, Nintendo has SUPPOSEDLY agreed to repair Lites with cracked hinges for free, even if it's out of warranty. See: http://www.engadget.com/2006/07/21/nintendo-fesses-up-to-ds-lite-hinge-defect-will-fix-for-free/ The comments here say it's kind of dubious, though.
This doesn't actually affect the hinge's action at all; what is actually cracked is just the housing. It doesn't really affect the integrity of the thing. If it doesn't bother you that much, you can live with it.
Nintendo's support line is 1-800-225-3700.