The Developer Tutorials blog has posted a series they've worked up to show bloggers out there running WordPress how to create a custom theme from scratch:

In this multi-part series I'll detail how to create and design a Wordpress theme from nothing more than your imagination using Photoshop, CSS, XHTML and PHP.

Here's the list so far:

• Part 1 - getting the tools and an overall layout idea
• Part 2 - splitting it up to make the layout parts
• Part 3 - converting the Photoshop template to XHTML
• Part 4 - cleaning up the XHTML and doing some styling with CSS
• Part 5 - how to preload some images with Javascript to make the page load faster
• Part 6 - creating the WordPress-specific markup
• Part 7 - working up the PHP functionality
• Part 8 - integrating PHP into the template
• Part 9 - marking up the "framework" of the site (header/footer/sidebar)
• Part 10 - adding additional files like a links page and changing the posting template
• Part 11 (?) - deploying the theme out to the public view