P.S. They also sell this one:
SL-LD210-F | CATEYE


The built in solar cell charges in 2 hours under good weather conditions and will power the Solar for up to 5 hrs. Available in both white and red for front and rear applications and comes with the FlexTight™ bracket.

If someone leaves their bike outside in sunlight, might be the thing for someone who's absent minded and doesn't remember to charge/change batteries...

Edit: The only 2 amazon reviews says that the light it puts out is to weak to really be effective though, unfortunately:
http://www.amazon.com/CatEye-Recharg...rds=SL-LD210-F