Ian initiated a new feature for WordPress themes that will be rolled out with version 2.7: Child themes. You can think of it as a cascading theme hierarchy.

For example, say you download a theme (parent theme) and then want to add a custom contact page. Instead of adding that custom page template to the main theme folder, you create a "new" theme (child theme) with only that one custom page and then link it to the parent theme. Viola! A complete, custom theme with the original theme left untouched, or in other words, dead simple to upgrade.