If you're following Matt's tutorial, you can't install from source as he does if you're on a PPC machine. To install it on a PPC machine, you'll need to download the .dmg and run the PPC .pkg installer. This is noted in the README file of the .dmg in the rb-appscript-0.3.0 folder. From that file:
"Please note that the version of Ruby included with Mac OS X 10.4 is missing the header files needed to build appscript on PPC-based Macs. You can avoid this issue by installing the latest version of Ruby from <http://www.ruby-lang.org/en/downloads/> and updating your shell login profiles to suit."
Just wanted to pass along the heads up.