If you want visual diff's while developing on OSX using subversion then this page has a good description of how to set it up and some scripts to wrap Apple's FileMerge application to give to nice visual merges/diffs.

I stole this image from the authors website just to give you an idea of what you're missing if you just use the text based merge tools.

The author doesn't mention it but rather then pass the path to the scripts to svn via the --diff-cmd option to can edit ~/.subversion/config and enter the full path to the scripts in the diff-cmd variable. Note it needs to be the full path as shell variables wont be expanded (so ~/bin/fmdiff will fail with error 255)

PS: I have nothing against the text based diffing tools so please don't flame me. After all I'm a Mac user so what do I know of suck magic.