Check out Apple TV. If you have an Apple TV in each location you can stream music to multiple locations. Used in conjunction with the free Apple Remote app on an ipod, iphone or even better on the ipad, makes for a very sweet system!
Here is the blurb from their site:
Your music. Your speakers. The perfect duet.
YouÂve got tons of music on your computer. But the best speakers in the house are part of your home theater system. Apple TV connects them without missing a beat. Your iTunes music library streams directly to Apple TV. So you can sit back, choose a tune, and enjoy the music.
Get more control with the Remote app.
Control Apple TV using your iPhone, iPad, or iPod touch. Download the Remote app for free from the App Store. And with a flick of your finger, browse your media collection. Remote uses the same easy-to-use interface you already use to browse music and video on your Multi-Touch devices.