Along with the wonderful Themeforest screencasts below, don’t forget that Chris himself has a whole site dedicated to WordPress – Digging Into WordPress. He has also written a book “Digging Into WordPress”, which as of September 1st, 2010 has been updated to Version 3! You can read more about the book and purchase a PDF version here.
The WordPress Codex is full of very useful information. While sometimes it can be a little overwhelming if you aren’t exactly sure what you’re looking for – it’s an invaluable resource! The area that you’ll visit most often is the Template Tags section. This page will outline all of the different WordPress tags that are available and how you can use them.
Themeforest, along with their PHP Screencasts, have started a WordPress one as well. This might be too good to be true! The WP screencasts will be taking the user through creating their own theme from scratch.