Monolingual.app will remove Intel or PPC code as well as unwanted languages.

From the terminal, ditto will also remove an architecture. For example to thin iTunes:

• cd /Applications;sudo ditto --arch [architecture to be stripped] iTunes.app iTunes-thin.app

(where [architecture to be stripped] is either "ppc" or "i386") (--arch ppc thins TO PPC)

Obviously, you want to do this on a fully backed-up system, because you're playing with sharp tools.