I read on another site that it was advisable to change some Malaysian Ringgit before you get to Kota Kinabalu because the airport is quite new and doesn't have money changing facilities yet.
It didn't mention ATMs though, so does anyone know if there's an ATM at Kota Kinabalu airport?
I can't answer you for sure, but I would recommend that you try to have some Ringgit lining your pocket when you get there. Having flown in and out of Miri (very clean and modern) prior to KK, I was kind of shocked by how big of a mess that airport was. Granted, I was there in October, but it seemed like their "rennovation" wasn't going to be completed anytime soon.
Just to be sure, if it's not a problem, I'd get some cash in KL or wherever you are flying in from prior.
Keep in mind that getting a taxi is also a bit confusing, but if you follow the masses out front, you should be able to figure it out.
If you're flying in to Kota Kinabalu airport via AirAsia, you'll be using the Terminal 2, the one that John described. I was on a stopover in KK to Lahad Datu (on work though) in mid-January, and the construction of that terminal had just been completed and was spanking new.
I couldn't remember seeing an ATM there (since I wasn't looking for one), but being an international airport, I'm very sure there is at least one from one of the major local banks. And like John says, it's better to have some Malaysian Ringgit with you in KL first, unless you are flying in from Bangkok.