Started a few months ago, and it's definitely not to do with sleep mode as I've checked but my MBA will drop the backlight then shut the screen off completely. Never had anything spilt on it, seems to occur most frequently when it's being opened (but that's not guaranteed). Hardware fault or something else? Nothing in the Console logs suggesting software weirdness.
EDIT: Because I was an idiot I didn't post the most useful info ;)
Macbook Air is a 2010 Core2Duo, the release just before the Core i series ones.
Running latest Snow Leopard.