Ironically enough, students may decide to stop learning when they reach shodan as they now know it all!
To sound horribly uncommitted to a strong opinion, I would say the phrase "you do not start learning until you are a black belt" has plenty of meanings depending on how it is said, (and who it is said by!)
In one respect, in many places you are not considered a 'serious' student until you've put in the time to get the coveted grade, but as an artificial measure of ability that can vary widely - I'd take that statement with a pinch of salt. In this context it also has an air of elitism about it, almost like saying "I'm better than you!"
The other meaning I'd say is that at shodan, you would have a grasp of the syllabus and techniques, and start to understand underlying principles and more subtle points that go beyond a set list of techniques.
Another thing that happens is the sudden awareness (and horror!) that you may be now setting an example - this (self imposed) awareness does bring about a change in attitude - for better or worse, which is in itself a form of learning I suppose!
I'll go back to the safety of lurking now..