I had been using Locomotive... but that appears to be terribly out-of-date (May 2007).
I had hopes that sakuzaku's script would do the trick... but it failed in several places. Afterwards, nothing worked. Honestly, I wasn't paying very close attention to it as it ran, and didn't crack it open to see what was going on... but that was my starting point.
I think the basic problem was that I had two other installations of ruby: one in /sw/bin/ruby, and the second in /usr/bin/ruby. ...So, after the script failed, I went and nuked those two executables and made them symlinks to /usr/local/bin/ruby.
But my gems were still miffed (I was trying to write a very, very simple script to go grab some XML from a web-app, and I was parsing it with hpricot). That turned out to be an easy fix:
export PATH=$GEM_HOME/bin:$PATHexport RUBYOPT=rubygemsexport RUBYLIB=$GEM_HOME/lib/ruby/site_ruby/1.8/
(clearly, this is in bash) ...And that did the trick.
Yes, yes, I know: I need to upgrade to Leopard. But I'm running on a G5, and I've heard the performance hit is considerable... so I'm dragging my feet.