According to the International Herald Tribune "Several American carriers, including US Airways and AirTran, recently started selling ads on napkins or stickers that appear on open tray tables. Over the summer, Ryanair, the European low-fare carrier, has gone further, installing advertising panels on the covers of the overhead luggage compartments and in the backs of closed tray tables."
Why don't they paint all of their aircraft rather like London busses do and become moving billboards. You could see all sorts of brands wanting to be on the side of an aircraft. Not of course that this is a new idea. Arguably the 'opportunities to see' are limited to when they are on the ground but with low cost carriers getting more and more like buses it seems the obvious thing to do