Updated Feb. 14, 2008
Deziner Folio has a really useful step-by-step guide to taking your XHTML/CSS layout and converting it to a WordPress theme. They say this whole process can be completed in 30-40 minutes!
Another option is DevBox’s Wireframe WordPress theme, which is available for download on their site. This is basically a barebones theme that you can use to easily take a XHTML/CSS layout and convert it into a WordPress theme.
