Sonos' wireless mesh music system is shipping: I just spent a few days with the Sonos system--two ZonePlayer units and a Controller--and wrote the linked review for Personal Tech Pipeline. My take is that the kind of person who might consider spending $499 per node for the best possible digital music system will love this. If $499 per node seems insane to you, you're not the audience.
There's so much care taken with almost every aspect of the system that once I overcame a networking problem--with my network, I should note--it was trivial to set up additional units, configure them, add music libraries, and so forth. It's price beyond my needs and means, but I can highly recommend it to those who want the best possible device currently available.
The ZonePlayer unit uses a wireless mesh system that is not interoperable with Wi-Fi, and, as far as I can tell, doesn't seem to interfere with Wi-Fi, either. You can connect up to 32 ZonePlayers in a network, but when you reach those numbers, the company recommends that several of them are connected to an Ethernet backbone to reduce the load on the mesh.