I've been spending the past couple of months in Tokyo. Worried about the radiation, I brought my DRGB-90 russian geiger counter /dosimeter that I had bought a couple of years ago. Unfortunately, the DRGB is a rather old analogue design and the readings at low (natural background radiation) are rather imprecise. I modified it so that it could hook up directly to an application on iOS called Geigerbot that is a sophisticated click-counter. Set up correctly, it can give you precise microsievert per hour readings. It also interfaces with Pachube which allows historical readings. Now Geigerbot can use the microphone on your iPad/iPhone to detect the geiger counter's audible clicks, but it will of course also pick extraneous external noise.
I wanted to directly interface my DRGB-90 with my iPhone so I could have more precise readings. Unfortunately, the DRGB doesn't have an external speaker jack or any other outputs. I wrote up an article (that I've since moved the actual hacking instructions to another blog) that talks about how to hack it. For this blog, here's the data coming out of the DRGB-90 + iPhone + Geigerbot.
It shows that radiation levels -- at least in my apartment in Tokyo -- is around 0.15 uSv/hr. This is actually lower by a full BED than background radiation in many places in the United States, which averages 0.23 uSv/hr. I haven't taken the unit mobile yet to see if there are any hotspots, but for now I feel less worried about the situation at least in terms of background radiation. I'm still concerned about food as a set of recent revelations make it clear that government and corporate monitoring has been less than ideal in this regard. Unfortunately, measuring food contamination is extremely difficult and not something a consumer can do herself. For more info, see safecast.org.