The OS X linker now understands -R, but distutils continues to pass the
wrong flags back in
distutils.unixccompiler.runtime_library_dir_option().  I'm checking with
the Apple folks as to exactly what the right flag is.

Do you mean the -rpath option?

Note that this is (IMHO) less useful  than on other systems because the 
linked-to library needs to have a specific link path (@rpath/mylib.dylib).

It can be useful for linking extentions though.

Using this feature requires OSX 10.5, AFAIK it is not supported on earlier  
releases of OSX.

Yes, we were looking at using this for linking PyLucene's JCC extension.
 I believe we came up with a different way of doing it.  It would still
be useful to have distutils.unixccompiler.runtime_library_dir_option()
updated to understand the right flags for Leopard.

Could you provide an example that shows why adding support for the -rpath 
option on OSX would be useful? (As in a set of source files that shows how 
this support would be used).

As I mentioned before the OSX support for -rpath behaves completely 
different than that on Linux, and I'd expect that adding 
runline_library_dirs_option support for OSX would cause confusion because 
of that.