I created a plugin that can be used to attach sensor nodes supporting the Contiki operating system to vera (see http://www.contiki-os.org/). I currently use the plugin to attach TelosB and Zolertia z1 (http://www.zolertia.com/products/Z1) nodes. Currently I use the light, temperature, humidity sensors that are by default on these nodes. I also modified one z1 node to read pulses from my gas meter and feed this into a default EnergyMetering device (translating gas usage to KWH). The sensor nodes use a 802.15.4 radio (2.4 GHz) and Contiki supplies a number of communication protocols (from MAC to routing) to go with it. The nice thing is that you have access to the os running on the nodes and the communication protocols used (and you can modify and change them as you like). In case anyone has a need for such setup please let me know I can share the code.
@utz, I’m very interested in this. I’m currently experimenting (or rather, I will be once all my parts come in) with extending my home automation using microcontrollers. I haven’t had a chance to read through the entire Contiki site yet, but it looks like the Contiki OS would be an interesting way to go (for one, it can be used with the MSP430 series of microcontrollers).
I have been looking at designing some small microcontroller based devices using wired ethernet to do some of the things you are mentioning (various sensors, tracking gas and electrical usage). I can do an Arduino based solution for about $22 with pre-assembled boards, but I also am looking at putting something together using a MSP430 or a plane AVR instead on a board I design. I have no particular reason to use ethernet rather than RF (802.15.4 for instance!) and I don’t really have anything in mind other than I figured I could find some use for a small army of microcontrollers spread through my house.
I haven’t designed a circuit or played with a microcontroller in quite some time so it is basically an excuse to do just that. 
An MSP430 board is good when you want to run on batteries. There are also some of the board designs available that are used by Contiki (Would save you some time). All available devices (also the ones I use) are around 80 to 100 euro (I think, have not the latest price in mind).
If you do not need to run on batteries you have more options. One might be to run a Raspberry Pi as sensor node. Hardware wise that is total overkill but the things cost only 25$.