It is possible, but just not practical. Most of the time the phone won't be in a position to get much power at all from a solar panel on the back of the phone.

You can get external battery packs with solar panels on, which you can plug your n900 into and use to charge it, and with those you could leave them in a position to get lots of sunlight, but even so, most pocketable ones won't recharge very quickly by solar power.