Kottke.org on the complimentary, but not necessarily interdependant concepts of XHTML/CSS design and semantically correct markup:

Coding web documents in valid XHTML doesn’t make them semantically useful nor does coding semantically correct documents mean the documents are standards-compliant; they are two distinct things but a powerful combination. As web designers, we need to be aware of what we’re getting with standards compliancy and semantically rich documents and that one does not necessarily lead to the other. More importantly, we need evangelize effectively to clients and budding XHTML coders & web designers, telling them precisely what’s so great about making sites standards-compliant and semantically useful and therefore worth spending money to redesign a site or time to learn valid XHTML/CSS.