So - if you paint your machine - get it done by like ColorWare etc - or put lots of stickers on your MBA and it has to go in for repairs - will Apple still fix it/replace the parts? Even if it is an external part that is covered by stickers/paint?
As mrrippey said, stickers should not void your warranty unless they leave residue on your machine which Apple could use as an excuse. Theoritically, glue does nothing to electric components as it is an insulator, but who knows! Painting the body of a laptop consists of disassembling all components, which obviously voids your warranty. Try contacting Apple, perhaps if the paintjob is done by ColorWare, the warranty will not be voided. Good Luck whatever you do!
From ColorWare:
If you purchase a new product from ColorWare we do include a 1 year parts and labor warranty that covers the coloring and hardware of the product. With a new product you also have the choice to upgrade to a two year warranty.
I think I rather disassemble myself and take it to an auto repair shop if I was in that much a need to paint it. It is aluminum so I am sure an auto body shop knows how to paint that. $500 is a tad steep just to say, I have a white, black or (pick your color) MacBook Air which you might not be able to resell. For $100 or $150, yeah......