IIRC it was on an AssemblerGames thread. Kinda don't have enough forum access to run a search though. They never let me register in spite of Satellablog.

EDIT: ah, so Segagaga released it. Here's some info I could find in a quick search.

http://www.gamesetwatch.com/2009/12/xmen_children_of_the_atom_satu.php

http://www.segagagadomain.com/saturn19/xmen-samp.htm

http://www.youtube.com/watch?v=L-BgZqVGIvM