I've been searching for ages for a good multi-room audio solution using raspberry pi and I've finally found one!

Logitech Media Center (squeeze server) running on either a server, nas or raspberry pi (anything that's always on)
http://www.mysqueezebox.com/download

Install SqueezePlug image onto each raspberry pi you want to be a media player (each room of your multi-room system). This image supports wifi dongles out of the box and is designed to run headless. Most importantly it comes pre installed with SqueezeSlave!

I've installed Squeezebox Remote onto my android phone and i'm away, I can select any source (internet radio, media files, iTunes, lastFM etc etc etc) and play to any location or multiple locations that I have a raspbpi!

Awesome and really easy to setup. Ultra cheap mutli-room audio!

Handy to know.
Just need to find a speaker that can be hacked & a Pi installed that can be used outdoors for BBQs etc

Agreed, I havent yet established if the ipod docks which charge provide enough current over the 5v line to run the pi with a wifi dongle attached, if so there are loads of options, if not there'll be more hacking involved.

This is gonna be perfect for my stereo. What kind of resources does it require? I'm looking to run it on my microserver with a few other bits as a NAS, media server, sab, sickbeard and all the like.

Just need the time and inclination to get everything sorted.