The One Arm Pushup is a fantastic exercise. It gives balance, strength and well to be honest it looks good!
Pavel's Naked Warrior book devotes virtually the whole volume to this exercise and the pistol (one leg squat).
Anyway, Jim at Lean and Hungry Fitness pointed to this great tutorial from Beast Skills on how to do a one arm pushup.
Beast Skills has some fine material and it is worth spending some time there; for example there are tutorials on the one leg squat, the handstand and even the Clapping Handstand Pushup!