While I am not part of the project - based on the fact the wikipedia link just goes to http://en.wikipedia.org/wiki/Lisp_%28programming_language%29 - I would assume that any lisp listed in the "dialects" in the sidebar would be a go? Meaning arc, scheme, clojure etc.
On the other hand, their wiki is hosted on "The Common Lisp Wiki", all the libraries they mention are exclusively Common Lisp, and their mailing list description says "The purpose of this list is to provide community to Common Lisp game developers."
But it's irrelevant in the grand scheme of things... The purpose of the thing is to motivate yourself to make a game; there are no winners or prizes or anything. So if you like, you can put your name on the list, and if your finished game is "disqualified" for being in the wrong language, you've still got your game, so you still win.
Oh great. That's all the Lisp community needs is a hoard of different Lisps competing to see who can write the coolest game using their particular dialect!
Hello folks, I've updated the compo site and wiki page to reflect that the contest is indeed open to any dialect of Lisp. Best of wishes to all participants!