Just to clarify:
You can install rb-appscript into Tiger's Apple-installed Ruby 1.8.2 using the binary .pkg installers in the .dmg. This avoids the missing header problem on PPC; there's also an i386 installer included for convenience. Folk who would like to try out appscript but don't have gcc installed will also find this the easiest option.
You can install your own copy of Ruby (the latest is 1.8.5) and then install rb-appscript into that from source or .gem without any problem, as long as you've got gcc installed. (If gcc isn't installed, installing any version of Developer Tools should provide a suitable copy.)
Apologies if my readmes caused any confusion.
p.s. Great article, Matt!