I think this experiment shows how subjective "celebrity" is. Granted, most people probably would not recognize Joshua Bell if they ran into him in a subway station... but enough have probably heard of him, and at least know that he is a famous violinist.

Had there been a large banner over his head, proclaiming, "Now appearing - Joshua Bell!", I would suspect that a larger crowd would have gathered - simply because it was Joshua Bell, the Famous Violinist, rather than some random-yet-talented busker.