Tokyo Hackerspace has developed a Geiger shield for Arduino, designed to work with the RDTN/Pachube radiation detector project. Akiba writes:
This is an Arduino-based geiger counter shield that makes it easy to upload data to the internet and also interchange tubes. Since it’s open source and Arduino-based, its also easy to hack this to other interesting applications.
You can read the dev history and view the schematics and board layout stuff here. You can also donate to the RDTN kickstarter. It’s been great watching these radiation measurement projects unfold — lots of talented people pooling their skills to solve a problem. I wish them all the best of luck!