If you have a Mac running snow leopard or later, you can create a multiple output device to accomplish this. Here's an article we wrote about how to set this up:
http://audioengineusa.com/articles/the-easy-way-to-get-multi-room-sound-with-bluetooth
http://audioengineusa.com/articles/the-easy-way-to-get-multi-room-sound-with-bluetooth
