In order to use the font scale mixin the modular-scale gem must be installed, it is not entirely clear from the documentation that this is indeed a separate step (gem install modular-scale). Also, require 'modular-scale' must be added to config.rb. Since font-scale is defaulted to true, the documentation should be updated and the require line should probably be added to the starter kit's config.rb.
Using compass is daunting enough for people who don't normally use ruby or the command line - and getting an error right away can scare people into thinking it is too complicated.
Yay for Compass + Drupal! Thanks.