Uni-Solar used to (or maybe still does?: http://www.uni-solar.com/products/commercial-products/pvl/in...) make flexible 3-mm-thick self-adhesive solar film, and if I remember correctly it could be installed on a roof without a permit, although it still required a permit for the electrical connections, so it probably didn't make things easier.

With some states only requiring UL listed inverters for small plug-in systems, it could become a more viable product.