I've recently started theming in Drupal. I've been custom theming in wordpress for clients since about November.

One of my client projects required switching a non phptemplate theme into one that uses the PHPTemplate Engine in wordpress. The PHPTemplate Engine allowed the user to switch the sidebars on and off at will and have the main content region expand and contract according to if there are sidebars or not.

Turning a theme it into a PHPTemplate engine just required transferring a template.php file into the theme. Enabling the features available in the PHPTemplate Engine does require a change to the XHTML structure of a page which includes inserting PHP functions. Not a time consuming task if one already knows where to place the functions in the XHTML structure of a page.