Unregulated solar panel can ideed fry your charging cirquitry by overvolting. You need one with USB out.

The biggets theoretical output one can expect from a solar panel small enough to fit on the back of the N900 is about 0.4W, i.e. about 108mA @ 3.7V (our battery's nominal voltage). That's assuming a rather optimistic 20% efficiency of the solar panel and maximum exposure to the sunlight (with constant tracking etc). You can see here how much the N900 consumes in various conditions.