It's a small circuit that automatically cuts off the power when the battery is fully charged. At only $1.59, it's a bargain! In less than a week from February 13, 2009 it should arrive.
February 22, 2009: The part arrived yesterday, and I just finished modifying the DS with the chip. It changed so much that I'll have to post a new instructable. I'll put a link to it right here when I'm done with it.
April 12, 2009: The newer version has been published! Go to http://www.instructables.com/id/Solar-DS-quotLightquot-Redone-and-Greatly-Impr/!