Take a fresh DarwinPorts^w MacPorts installation. Install ruby (sudo port install ruby) then install Ruby Gems. MacPorts resolvs dependencies automagically like any decent package manager and...
golem:~ brianm$ sudo port install rb-rubygems
---> Fetching perl5.8
---> Attempting to fetch perl-5.8.8.tar.bz2 from http://www.cpan.org/src/5.0/
I love it!