My ruby version is: ruby 1.8.6 (2007-09-24 patchlevel 111) [i686- darwin9.2.0] ... The gem is installed at `/usr/local/lib/ruby/gems/1.8/gems/rb- appscript-0.5.1`
As Allan pointed out, you do appear to have a custom Ruby (the stock platform string is [universal-darwin9.0], not [i686-darwin9.2.0]). Same compile date and patchlevel, so it's easy to mistake.
Just a shot in the dark, but perhaps you followed Dan Benjamin's (hivelogic) guide? That would certainly explain the /usr/local/lib path.
I just tested installing rb-appscript with stock ruby and gems; `gem which appscript` yields:
/Library/Ruby/Gems/1.8/gems/rb-appscript-0.5.1/lib/appscript.rb
Alternately, is it possible you have a leftover $GEM_HOME environment variable from a previous custom installation of ruby and/or gems?
HTH, Daniel