Retrofitting is pretty difficult to do for HA -- I'd still be happy with bricks even, my walls are just solid stone in many places (great for keeping the place cool during summers though).

Have you tried RF for a retrofit like ZigBee or Z-Wave - they're pretty robust with mesh networks, can link to powerlines where possible and do have a decent protocol designs compared to a lot of other stuff people try to automate their homes with.

If you're interested in discussing how to do things from scratch, would really appreciate your feedback, notes or comments on your experiences on OpenRemote website.

Our goal is to build an open system for home automation with the ability to go from do-it-yourself model to pre-built or professionally installed setup. While waiting for open specifications (802.15.4, 6LoWPAN, etc.) to emerge, we are trying to integrate as many legacy HA systems as possible, placing an Open Source OpenRemote system as an integration layer between proprietary protocols. Currently focusing on X10, Insteon, KNX, UPnP, GC, ZigBee and Z-Wave protocols, would be interested in knowing if you have others you'd consider important to work with.

Our goal is to enable commodity hardware to be used with home automation wherever possible to reduce some high-cost points in usual installations such as buying vendor-specific wall panels. We work with iPhone, and Android-based smartphones as control devices to create integrated control panels that allow single point of control on heterogeneous systems. This is often the most highly prized and fattest margin for the existing industry players as they know that integrating HVAC, lights, A/V with all proprietary protocols leaves customers and users with little or no options.

OpenRemote is an effort we've started focusing on home automation within Open Source space.

The goal is to bring open system and open protocols to all users with the ability to go from do-it-yourself model to pre-built or professionally installed building and home automation. While some open specifications are emerging (802.15.4, 6LoWPAN, etc.), the current state of the industry is built around proprietary protocols and hardware. While waiting for things to improve, we will integrate as many of the legacy HA systems as possible, placing OpenRemote as an integration layer between various existing systems (Open Source is good at integrating with its "scratch-my-itch" model).

Our goal is to enable commodity hardware to be used with iPhone, Android and Linux support, especially to build integrated control panels for homes that allow single point of control on networked devices at home. This is often the most highly prized and fattest margin for the existing industry players as they know that integrating HVAC, lights, A/V with all proprietary protocols leaves customers and users with little options. A 15-inch wall panel in the HA industry can literally draw $5000 to $7000 price tags when the cost to manufacture is a tiny fraction of that (imagine the size of a TV you could buy for a $7k).

The Sheeva Plug mentioned elsewhere is one option.

Another we've had a good experience with are the ALIX boards from PC Engines. The ALIX 3D3 board we run is at 3 watts at idle, up to 5W (not including whatever you attach via USB). It runs from CompactFlash and has no fan so keeps the energy down nicely. CPU is AMD Geode. You get serial, WiFi, Ethernet, USB connectivity and VGA port if you need it for a screen.

We've used it with the Voyage Linux distribution (Debian-based) and it's running great. Building from parts the whole box costs you around $200 or less and you get a full-fledged system with which to tinker with. We've got a bill of materials online and some build instructions as well.