I came across an awesome application for the Mac today. It utilizes the AirTunes features of my Airport Express and allows me to broadcast anything (not just from iTunes) from my computer onto the AirTunes speakers or the speakers of any computer connected to the Airport Express. You just need to download the client program onto the desired computers to allow use of their speakers.
It’s called Airfoil, by Rogue Amoeba, and it can be found here. I am currently enjoying Sirius radio over my home speakers. Who needs a home docking station?