I am really loving the improvements to subthemes in Drupal 6. They took what was an interesting idea in Drupal 5 and made it even more powerful and much easier to use. I have been finding a lot of power rolling out a sets of websites, using subsubthemes.
Theme X: Free theme downloaded from drupal.org
Theme Y: My modifications to that theme, as a subtheme of Theme X. I want these mods to apply to all my subsubthemes. base_theme = Theme X
Themes A, B, & C: The actual themes used on my sites. base_theme = Theme Y
The great thing about this setup is that I can continue to install updates to Theme X, with minimal disruption to my actual sites' themes. There was one problem though: