I just got my free iPod Touch with my new Mac Pro two days ago, and I noticed this, too!
It was my understanding that there's no external speaker, yet when I press the power/sleep buttom there is an audible "click" that does not at all seem to be a noise the hardware is physically making.
I don't think it has happened any other times when pressing home, etc. Does this have a logical explanation and we're just being dorks?