Hi bk
You are correct, a setting is available in the custom functions to disable the halfway press on the shutter release button for the AF lock (this also applies to AF-C as well as AF-S) this setting disables the halfway press on the shutter button and transfers AF lock solely to the AF button (this links me to point out another typical example of reviewers not getting their facts right, see note below*)
In addition the new firmware update for the K10D has added another feature, A further custom function can be set for the AF button, this enables you to have the focus point set to auto (multipoint) or even user selected, but pressing and holding the AF button revert focus lock back to centre point (until released)
*A quick note on reviews, Typically another "assumption" by those who dont really know what their talking about, A leading UK photo Magazine recently said (in so many words) that the AF button was pointless as it did nothing that the halfway press on the shutter release already did........errr, well I beg to differ as the custome function setting above reveals, anyone with unsteady fingers would be firing off frame after unwanted frame if they couldn't control the halfway press on the shutter release, hence the addition of a seperate AF button on the camera, programmable to be used or not.
I have found the AF button invaluable when trying to focus on difficult subjects, the ability to keep a thumb on this button and lock AF while not having to worry about applying to much pressure and the shutter firing.
My Pentax istD had this feature and I have been using it for the past three years, brilliant!