A lot of people have been asking about syncing Rios up to play simultaneously play across the house. This wouldn't be the perfect solution but couldn't you use JReceiver and create a shoutcast channel to play your songs and then have all the Rios hit the shoutcast channel through JReceiver?
Now you wouldn't be able to pause or change songs but this would allow you listen across all Rios wouldn't it? I would try this but I only have one. Just thought I would throw it out there.