Rogue Amoeba has announced an update for Airfoil to version 3.1, adding support for sending audio to the Apple TV. Unlike the built-in iTunes support, Airfoil allows you to stream any audio source from your Mac to your Apple TV, Airport Express, or other Macs running their Airfoil Speakers client application. This allows you to use your Apple TV to listen to web-based "radio" sources like Pandora (my favorite) or last.fm, as well as other media players like QuickTime Player, etc.
Airfoil 3.1 is a free update to registered users and $25 for new users. A demo is available for download.