Lebron James is fulfilling what he said some time ago. Number 23 should only be MJ's number. It is said that he has already applied to the NBA to wear number 6.
Being regarded by many since his younger days to be the next basketball great, James can not help but be compared to MJ. All the more when he wore jersey number 23 when he turned pro.
I guess it's a class act to pay tribute to greatness. And it shows that James doesn't want just to be called the next Jordan especially that he has yet to win an NBA championship. He wants fans to witness his very own greatness.
I would guess that Lebron chose 6, a number he wore during the olympics, because it is his the first major championship that he won so far in his career.
It remains to be seen though if other players using 23 would follow James' act.