Ha ha, love it! I hope it works.
In the U.K, the dealer will also wash the car as part of the service, unless you ask them not to. I would not trust the dealer to wash my car with their bucket of dirty water and gritty sponge - I don't want them to add complementary swirls to my paintwork
So I hang one of these on the rear view mirror;
http://carboncollective.com/product/do-not-wash-pvc-hook/And I tie a tag on my keys with 'please do not wash this car' written on it, and also ask the person at the service reception to write 'do not wash this car' on the service worksheet.
It's worked so far.