At some point while developing for the iPhone or iPad you'll undoubtfully need to create a button with some odd image and notice it will not apply rounded corners to it. It just won't clip y default. Luckily there is a relatively easy fix for that in version 3.0 and up.
CALayer * layer = [myButton layer];
Do not forget to #import <QuartzCore/QuartzCore.h>
Hope it helps!