On my 50D I have it programmed so that I can use either the AF-ON button or the shutter release to auto focus. If I don't want the shutter release to "over-ride" the AF-ON buttonI have to hold the AF-ON button while hitting the shutter release. I forget what custom setting this is, but for me I like that I can still use the shutter release to start AF if I want to without having to go into the custom menu.