SiliconFarmer found this awesome little solar cell on Digikey that puts out 4V at 50 microamps. Not a lot of juice, but add a 0.33F supercapacitor, and leave it in the sun for about 7 hours, and you have enough energy to run an Atmega168V in active mode at 1MHz for about 22 minutes, or in sleep mode for 4 days (!).
Solar Cell: CLA289-ND on digikey
Super cap: P11064-ND on digikey
Diode: MMBD914-FDICT-ND on digikey
PCB from the DorkbotPDX PCB Order