The Mac OS X developer tools includes FileMerge, which is a pretty nice file comparison tool. When I wrote the subversion diff wrapper script, I wanted to be able to use FileMerge as the diff tool, but I wasn't able to get it to work.
I didn't realize that opendiff was a provided command line tool that allow this to work. Out of the box, when I configure the wrapper script to use opendiff, one instance of FileMerge is opened for every file that has been modified.
This hint on macosxhints, describes a way to have opendiff wait for a diff to be closed before the next file comparision is started.