I was looking over the wikipedia article for the 737NG when I came across this picture:
As you can see, the operator of the jet has written their companies web address on the winglet. Which seems like a waste, I'm already on their airplane, there's no need to sell me, it's not so much an ad as a reminder at that point...
So, I couldn't help but think, when looking at it, that it would be somewhat clever if RedBull bought the rights to advertise on a 737NG and wrote something on that spot that said, "Redbull gives you winglets" (rimshot).
Sillines aside, there is a serious question here. Generally speaking if there is a public surface that many many people will be forced to stare at for any length of time, then there is an advertisement there. Bus stops, billboards, the sides of buildings, there are ads virtually everywhere. Heck, commercial aviation equipment even has a few already with some of the jetways you see at airports:
There are tens of thousands of people who see winglets every day (and even more who see winglets in travel pictures, blogs, etc), so why aren't advertisers taking advantage of the space and, further, why aren't airlines trying to sell the space in order to increase revenue?
I'm wondering if perhaps there are regulations that might make this difficult? A good advertising spot will change images and products fairly frequently. On a winglet that would necessitate either a new paint job or, perhaps, a new vinyl wrap (like what is used in NASCAR.) And I'm wondering if the regulations for commercial aircraft regarding how the winglets are painted would prohibit this sort of "quick switch" activity (are wraps even legal)? Or perhaps there is a certification process for all new paint jobs that would make it so the amount spent to advertise would never match the increase in sales?
Or is it just that airlines have a sense of pride about their aircraft and respect their passengers enough to not bombard them with ads? (...yeah, I doubt it's this last one as well ;-) ).
Edit: I did mark this one for FAA Regulations, but I feel EASA regulations should be similar enough to give some insight as well, so if you want to answer with those regs feel free.