I finally decided to plonk down $29 and install Mac OS X Snow Leopard. When I tried running Cucumber, I got an error about the Gherkin lexer... "gherkin_lexer_en.bundle: mach-o, but wrong architecture". It turns out this was caused by the upgrade to Snow Leopard.
I had to upgrade mac ports, delete all my old gems and reinstall them. I presume vendorized gems will need to add a new 64-bit version.
Here are some links I found useful in this quest: