http://www.househacker.com/ househacker/tags/lawnmower
This how-to walks you through the process of modifying a mower as well as creating the solar panel/charging station that provides the power. The mower does take a while to charge up (40 hours by the authors calculations, assuming a half-charged battery), but since most lawns don't require mowings of any greater frequency than that (or anywhere near that, one would hope), the solar solution seems like a perfect fit for the task.

The author provides a lot of detail, including a couple of schematics and a parts and costs table. The final price tag was $699, with the bulk of the cost going towards the motor ($312), solar panel ($120) and battery ($90).]]> http://www.househacker.com/permanent/DIY-Solar-Charged-Lawnmower