Revisiting this topic, I did see where Toyota was testing a new solar system on the Prius plug in using more efficient/expensive solar cells covering the hood, roof, rear window and trunk surfaces. It produced a respectable amount of power in good conditions. While it may not be cost effective and practical at the moment, it may be worthwhile at some point in the future.
Thinking through theoreticals, the engineering and materials, and therefore cost, to have a long lasting solution that works on a car is going to be challenging. Think about a solar installation that has to handle everything a roof mount system takes (wind, hail, temp extremes, ice/snow) and now add car washes, vibration and shocks from driving on all kinds of roads, road salt in the winter, impacts from insects/debris, scrubbing with cleaners/waxes, and all of that happening for years to the car mounted solar system. You'll have to keep the car clean to not hinder production and that will subject the panels to cleaners and scrubbing that roof panels wouldn't see.
It will take a lot of testing to make sure that it can work and reliably generate meaningful power over the life of the car. If it adds a lot of expense to the car, insurance expense for repairs that require specialized repair centers isn't going to be cheap either.
Again, not that it's impossible, but I wouldn't count on a viable and affordable solution any time soon.
