www.mcmajeres.com the website itself is a bit ghetto but the software definately is not.
Basically what you download is an iremote server on your desktop and the client on your handheld (looks like any other MP3 player. The server knows where all your music is, and sends it to the iremote client. You choose the music you listen to just like you would in internet explorer. You have to have to use winamp on your desktop. It basically just sets up different "instances" of winamp. When you choose what room you want to listen to music in - the server sends winamp some sort of script to call up the appropriate instance of winamp. Then you just choose what you want and can control the equalizer, sound, make playlists, etc etc.
May not be the best articulation of it, but it's all still kind of new to me, but i think that's relatively accurate.