Good question Philosoraptor!
I've always wondered this myself. And what about the afternoons?
Not exactly sure how the exercise "good-morning" got it's name, but according to Wikipedia:
The movement resembles bowing to greet someone "good morning"
I've never had anyone bow like this to me in the morning, but it seems common in Japan, with a shorter range of motion. And I bet they bow throughout the day, not just in the mornings.
