A Nintendo spokeperson says: "In the U.S., the reported number of small cosmetic cracks in the plastic hinge of DS Lite systems represents less than 0.02 percent of the total units sold. This cosmetic issue in no way impacts the gameplay or integrity of the DS Lite. Nintendo stands behind the quality of our products and encourages DS Lite owners to contact our Customer Service Department if they are not happy with the functionality of their systems."
Update: Nintendo will fix cracked DSs for no charge: Link