@Vodden I also am doing a Reno and have the option of putting in cabling throughout the house. As I already have POE camera's to install I have a POE switch. So was looking into options of at least powering the sensors with POE.
I have based my sensors around the Nano so wanted to convert the POE output to USB voltage and ideally plug the sensor in using a USB cable.

After some research this seems to give me the POE to 5V for $13
http://www.pccasegear.com/index.php?main_page=product_info&products_id=26091&cPath=1657&utm_source=googlebase&utm_medium=cse&utm_campaign=export_feed#googlebase

The sensor package itself will still be cheapish as most of the arduinos will be running more than one sensor. It is also cheaper and easier than using an Ethernet shield and having to hack it onto the Nano's

All I need now is to find a cable with a jack on one end to a USB mini. Anyone have any ideas?