I'd forgotten about this thread until now, but thought it would be no harm to post a quick update.
I briefly looked at Netstreams as an alternative to A-Bus, but it was way more than my budget would allow and even at that, I'd still need to adapt the wiring.
After looking at the Sonos system in more detail (and getting a good demo from a friend who already had it installed), I decided to bite the bullet and adapt the A-Bus wiring to work with the Sonos.
To recap, all the main bedrooms and living spaces in the house had wallboxes at head height, containing dual speaker cables running to overhead speakers in the same room, and a CAT5 control cable running back to the attic. The downstairs rooms and master bedroom also had an additional CAT5 cable running from the wallbox to another wallbox in the corner, near other A/V sockets, which was intended to be used to feed a local line-in to the A-Bus.
Making the Sonos work with this turned out to be fairly straightforward. For the downstairs rooms and master bedroom, I used the line-in CAT5 cable to extend the dual speaker cables from the control wallbox to the corner of the room (one pair for each speaker wire, four in total). Hardly ideal, but the cable lengths are quite short - only around 4-5M - and it saved having to open up walls to pull new cable. The CAT5 is terminated in a normal dual speaker socket in the corner of the room, and a blanking plate covers over the control wallbox.
A standard Sonos ZP120 sits in the corner of each room and connects directly to the speaker sockets, which then feed the ceiling speakers. Having the ZP120 in the same room is convenient for visitors/babysitters/etc since it means they can easily adjust or mute the volume without needing to learn the Sonos remote.
For the upstairs bathrooms & other bedrooms, I spent a bit more effort to pull new speaker cables from the ceiling speakers to the attic closet, where all the CAT5 was terminated. Conveniently, all such rooms had at least one speaker location which was close to an outside wall of the house. By removing the speaker and reaching into the void, I was able to feed the new cables into the eaves of the attic, around the edge of the attic flooring.
This got me a home run for one of the two speakers in the ceiling. For the second speaker, I bridged to the existing cable running from the speaker down to the in-room control wallbox. Within the wallbox, I then connected that speaker cable to the cable for the other speaker, and hey presto, I now had both speakers with home-runs back to the attic.
It worked pretty out well, so now all the rooms in the house are fully Sonos-capable. Even for the rooms with CAT5 bridging the speaker signals, the sound quality is very decent. With young kids in the house, high-fidelity audio was never going to be a high priority! Our kitchen is quite big and has two pairs of speakers briding over a single CAT5 cable, and even wth that, the sound is very decent.
Meanwhile, all the CAT5 control cables in the walls are left unused. Since this seems like a waste, I've bought a power-over-Ethernet midspan hub to go in the attic, which will allow me to feed PoE to each control point (no actual Ethernet, just the power). A small in-wall splitter then recovers the 48V power and converts it down to 5V at 2A, which is exactly what the Sonos ZP200 controller's charge cradle requires.
Right now, I'm working on a decent way to adapt the ZP200 charge cradle for wall mounting. The catch is that it needs to maintain a slight backwards tilt to prevent the controller falling out onto the floor. I've recently bought a pack of this stuff -- http://sugru.com/
- which looks like it might give me a neat way to mould a support for it. (I had hoped Sonos might announce a wall-mounted cradle with built-in PoE support, but a year on, no sign of such a product.)
If I get this working, then I can have most of my ZP200 controllers wall-mounted and self-charging, without lots of trailing power bricks in each room.
In summary, very happy with the Sonos system! Would definitely recommend.