Drupal 7 Theming: so awesome you'll need a change of pants
The changes to the Drupal 7 Theme layer aren't revolutionary, but there are so many improvements, you may have trouble... ah... controlling yourself.
Numerous name changes (like sidebars and menus)
phptemplate_ prefix is gone
$classes and $classes_array
theme functions and preprocess functions
More regions, less hard-coded page variables
Holy crap, content is now a block!
template suggestion wildcard