iPod Nano wake screen sometimes shows music instead of Clock
Might as well answer this.

I've found out that it depends on what screen you had active before the iPod goes to sleep. If it goes to sleep with the Music screen, that apparently gets priority over the clock on waking (even if you have checked the 'time on wake' setting).

A workaround I've found is navigating to any Settings screen before putting the iPod to sleep. Upon waking up, he'll 'default' back to the Clock then... Apparently the Music screen has a higher 'priority' than the Settings screen, go figure.