We are all fascinated by X10, and if you want to go deeper in the subject, check the blog  Arduino X10 Collection.

There is a two part series in the making of a DIY Geiger Counter Project which is built with an Arduino Microcontroller.

I put it all into an old laptop power supply case – not my best work, but as we said in Arkansas, “it ain’t no piano”. It does have a nice sturdy feel though.
I’ve always been fascinated with measurement tools, so building a Geiger counter seemed like a logical thing to do. I will describe the build process here – even though the Arduino only plays small part, and that only a truly sick person (which I guess I am) would consider a Geiger counter as part of a Home Automation project.

via [Arduino X10 Collection]