According to the Muhyojo Website (via Anime News Network), Spider-Man and X-Men co-creator Stan Lee is collaborating on a new project for Shueisha with Shaman King creator Hiroyuki Takei (Shaman King is published in the U.S. by Viz Media).  Together they are creating an as yet unnamed story that will help to launch Shueisha's Jump Sq. II (Jump Square Second), which debuts in Japan on April 18th. Jump Sq.II is a spin-off of the Jump Square manga anthology -- and the announcement of the Lee/Takei collaboration was made in the April issue of Jump Square, which was released on March 4th in Japan.

While there have been manga adaptations of both Spider-Man and X-Men, this is the first time that Stan Lee has worked directly with a Japanese collaborator, though the exact nature of the collaboration is not known at this time.