see if you have one lying around. I almost never use the one on my iBook.
I'm pretty sure this is a bug with the store: the Macbook Air doesn't seem to have an IR sensor.
edit: I'm wrong:
http://developer.apple.com/document...okAir_0801/Articles/ProductDeveloperNote.html
you can actually see the ir sensor on the front to the right of the pulsing sleep light in the mba ad. there's a really good shot of it at the beginning.