Until there's a significant leap in solar panel technology, there's no possibility of coming even near meeting the 10W power draw with a solar panel the size of our dell minis.
The typical maximum solar insolation at ground level is about 1000 W/m2. The mini's surface area is about .04 square meter (232mmx172mm), so we need at least a 25% efficiency solar cell to generate 10W. If we take account of inclination angle, loss due to power conversion, storage and voltage regulation loss, the efficiency have to be even higher. This doesn't even account for the fact that we'll have to constantly change the instance angle of the solar panel to ensure maximum insolation. Additionally, the voltage and current output will depend strongly on local conditions and will require some pretty complicated regulator circuit.
It's probably easier to just pack a spare battery. Looks like that's what the package includes, anyways.