You know what I find kind of annoying, the outer aluminum case of the Air attracts a lot of dust and such, especially the top when the laptops been closed for a day.
I also dislike how the palm rest area gets this white-ish stain from natural perspiration while working/typing on the laptop that I have to constantly wipe off.
=T
Anyway, like others, I use a micro fiber cloth with a tab of water.
Why not consider a case for the palm rest (like this one: http://twelvesouth.com/products/surfacepad_air/ )?