Definitely a nice touch!
At the moment I am installing 64 solar panels on the roof of my company (11.500 Wp), which will be used to charge my Karma (and make the building self-sustaining). I've been designing - online - a sticker to put on the back of the car, with the text 'solar powered' (or 'electric'). I found an italic font in a light green color that resembles the one used in EVer, so it will fit nicely.
The problem I have with the EVer-sign is that it doesn't show the car is an electric car. Most people will think it is just another, rather nice looking gas guzzler, which is a shame as the thousands of Karma's will be frontrunners, that are supposed to make people enthusiastic about electric cars.