The Libelium Team have created a Geiger Counter (radiation sensor), and shipped the first batch to Japan's Tokyo Hackergroup and other people helping in the relief effort. The Arduino sensor board can sense radiation all the way up to cancer inducing levels. In the video Vaseline Glass (Uranium glass) was charged with ultraviolet light, which allows for a few minutes of weak radioactivity. Complete schematics and source code can be downloaded from their site, or you can buy one of the kits.
A new wireless versions allows for the deployment of these sensors that is read remotely via Zigbee and GPRA radios (Waspmote platform). I believe this wireless option will be the most useful. Blanket an area with them to get a full map of radiation spread or decline. And Libelium's boards are just a fraction of the cost of other radiation monitoring equipment. Useful, if the authorities that be do not mind using un-certified equipment.
It seems supporting the Hackergroups and youth in Japan is a good idea. Unprecedented numbers of young Japanese people rushed to volunteer their time in cleanup efforts. Disasters tend to motivate people to help, but there is more to this altruistic movement. Japanese in their 20s are experiencing a work world that has not existed in quite some time. Job security is a thing of the past with their economy, and unemployment is high. Where they can not help with financial donations, they are instead offering their time. Unlike any time before, social media is linking up likeminded people and organizing relief efforts. Just look at Libelium's aid, through social media their boards are getting to people who need them.