The Advanced Theming in Drupal 7 series picks up where the Theming Basics with Drupal 7 left off. The first two videos, Introduction to Advanced Theming in Drupal 7 and PHP for Themers, give a high-level overview of the the template.php file and a review of the basic PHP you need when working with Drupal themes.
The whole series will cover the new page rendering system, and shows you the process of iterating through the theming process with the use of various debugging statements and the theme developer tool. You can start using advanced theming concepts such as: how to theme forms, how to cleanly expose information to the theme system, and how to empower your site administrators by making certain parts of the theme available to them for customization. You'll also learn more about some of the new theming hooks that are new in Drupal 7. By the end of the series, you'll have enough tools to be able to completely control the HTML, CSS and JavaScript that Drupal outputs.