I use a Kensington Pocket Presenter with my OS X laptop and it works great with S5. In fact, I’ve already hacked my personal copy of S5 to have a “blank screen” feature as a result of using that remote. Basically, any remote that works with Powerpoint or Keynote is very likely to work with S5, because so far as I can tell they all send keyboard commands to the computer, so it’s all good. See the comments on http://meyerweb.com/eric/thoughts/2006/05/04/presentation-remotes/ for more recommendations from people on good remotes to use.
(Related note: S5 has now moved to http://s5project.org/ and will hopefully start adding features again.)