Where are the best resources for learning how to create Drupal Themes from a visual design?

I don't think there are any resources specific to creating Drupal themes from visual designs, but a Drupal theme is after all just HTML+CSS with a sprinkling of PHP, really any resource that helps slicing up visual designs into HTML and CSS should be a help.

For the Drupal side of things I'd say the best thing is just to dive in and try, you'll learn better by doing than trying to follow tutorials, even though you may lose some hair in the process. Choose a side project to start with, not something with a looming deadline!

Drupal Dojo has some lessons on theming

Books, probably most useful for learning the Drupal specific aspects of theming, such as regions, blocks and phptemplate: Pro Drupal development Front End Drupal

Packt do a whole range, including several relevant to themes

Training, if you need hands on help:

Reference resources for Drupal theming: The Drupal theming guide Drupal API reference, particularly the theme section Unofficial contrib module api reference

Don't forget the community if you need help with a specific question: http://drupal.org/irc http://drupal.org/forum

Most of all you'll need determination and patience. Good luck.