Our buddies at the Consumerist got a nice tip from a disgruntled Apple Genius on how to drop off your Mac for repairs without waiting. If you can't be buggered to make an appointment, all you have to do is show up, keep refusing to come back at a later date, and they'll eventually give you a Quick Drop form to fill out.
Once you do, you leave your computer with them and within a day or two you'll get a call from a Genius with the diagnosis and a quote (which is often $0 if you're under AppleCare). Not a bad way to get your Mac repaired if you have to work all day and can't make it in to a Genius appointment.
Don't Wait For A Genius: Quick Drop Your Mac And Go [Consumerist]