When I learned I couldn't stream Pandora through iTunes to Airport Express, I was bummed. Luckily, a quick search found Airfoil with tons of positive reviews by Mac users.
5 minutes after downloading Airfoil, I was streaming Pandora to my stereo.

Airfoil is amazingly simple to use ... just click the speaker icon and listen.

This is one of those cases where I feel I got exactly what I paid for and would recommend it to anyone who wants to bypass ITunes and stream web based radio directly to Airport Express.