Finally re-found this piece by Dan Milligan... it was posted on the conceptart.org website a while back as part of their Thunderdome 5 competition. The theme of the competition was to redesign classic game characters, and Dan chose the Marathon Marine for a makeover.
Very different take on the character, and I for one like it a lot aside from the squinty little eyes. Dan's website has some pretty impressive work on it, but AFAIK this is his only Marathon-related piece.
Dan's Website:
http://homepage.mac.com/danmilligan/Menu3.html
Original Thunderdome Thread (cool re-thinking of videogame characters):
http://www.conceptart.org/forums/showthread.php?s=&threadid=5979
JF