From STARTERKIT.info.txt (Zen 6.x-2.0):
; You can also override any of Zen's stylesheets or any module's stylesheets, ; an /extremely/ useful feature. See the excellent Drupal 6 Theme Guide at ; http://drupal.org/node/171209 for more details.
This should be updated. The overriding info at that URL was moved on December 1, 2008 to http://drupal.org/node/263967 ("Overriding style sheets from modules and base themes").