iPods use emissive displays (backlit LCDs) at a high refresh rate which use far more power than the low refresh rate entirely reflective displays on most watches. If the display on the iPod watch was always on it'd have unusably small battery life.
Right, I understand. My point is that iPod Nano just doesn't seem to work well as a watch. If Apple were to design a watch, I don't think it would require people to push a button to view the time.