module::function looks good to me. Someone else suggested allowing user-defined syntax characters; this would be a good use-case. It only has to expand into (module 'function), so the implementation is basically trivial.
Why not module:function? Abuse the property that : stands for composition. A more lispy solution of (module function) might also work, and its only an extra character. Though if I were going for an Arc solution, it should probably be (bin fn) or (md fn).
