Using variable_get() in multiple places, without duplicating $default When using variable_get() in multiple places, how should one avoid duplicating the $default values?

define()?  Or is there some built-in Drupal-specific mechanism I'm missing?
There is no built in mechanism. In fact, there is a long standing issue to add one http://drupal.org/node/145164.

Defining constants, as you suggested, is the only method I've seen to make it a bit cleaner.