Clark Kent’s secret identity is Superman.
Everyone should at least know that.
However, I believe it is better said in reverse:
Superman’s secret identity is Clark Kent.
Superman becomes who he really is when he is himself, Superman.
When he “becomes” Clark Kent, Superman humbly conserves himself, and brings himself to a level that is less than a super human. Not only that, he is also “disguising” himself by becoming something that he is not, which is a human.
Why does he do this? Because his character development permits him to do so. He was raised as a Human (even though he is not), and thus given the traits of a good and lawful character.
It’s easy for him to be super all his life… but to be normal… it was much harder for him than it was for anyone else. I believe that was one of his biggest challenge and obstacles.
Superman is truly super when he becomes normal like us.
[Update of October 18, 2011] It seems like I written this article @ dotSpiral: Why Superman is Super. Looks like my ideals has stuck with me for a year :]