Yes, I understand - much like the entire-mouse-button on some Apple mice. The concern is, moving the cursor and clicking the button have always been and should always be two distinct hand motions, so that one never accidentally does one when intending the other.

If Apple does the mechanics well, it won't be a problem - but it's an easy one to mess up.

Jason Snell over at Macworld reports that the trackpad is smart enough to realize that if you have your thumb at the "bottom" of the trackpad and index finger in the main portion it will sense the click with your thumb and pointing with your finger separately without being confused.

This obviously doesn't cure the confusion of the user who doesn't try this and gets frustrated looking for a button!

They'll probably have to add a message, "Welcome to OS X! CLick the bottom portion of the trackpad with your thumb while...