no, gox is still the leading exchange. We saw it in every major increase / decrease in the last weeks. Every exchange waiting for gox to break a wall, seconds later all exchanges follow.
It looks to me like MtGox is following the Chinese exchanges right now. This latest increase for instance, follows the Chinese markets. Also, see the increase in volume and how it follows.
It did lead in the latest mini-crash, but that was 2 weeks ago and things seem to have changed drastically. MtGox is not even the second biggest exchange in volume right now.