making themes is pretty straight forward as to making website layouts... meaning it is the same method of design...

i think with wordpress themes, (after you are done with the design PSD) you need to create a static HTML+CSS template of each page... This is to help with the "development" process. Creating an HTML file for every template, allows you to test it across all browsers, (validating both HTML and CSS markups) then all you need to do is cut & paste the WordPress code. As to the code used to display the Wordpress theme, you will need to "deconstruct" the WordPress default theme and see how it is structured.

scott

there are progs out there (like Sitegrinder  -  PSD to WORDPRESS) to make life easier (but at a cost!)