willem: Even if you don't add everything and the kitchen sink to your editor, a simple viewer can be very useful sometimes. It can be nice to just peek around a map without having to wait 5 minutes for radiant to load, or stop everything else you're doing to boot up quake. It will be a universal binary I hope? ;)
necros: gtkradiant runs on mac os x:
http://redsaurus.net/gtkrwiki/index.php/Downloads (ignore the wiki spam at the top of the page)