We were discussing this very issue (NYC’s move to crack down on air monitors without a permit and all the ways you could get around this) at the recent Make: DC planning meeting. Bob Funk writes on HackedGadgets:

OK, NYC has it’s reasons to be a little careful but Richard Falkenrath, the NYPD’s deputy commissioner for counterterrorism may be just a little bit too careful. As the Village Voice reports that the city is just a little worried that the good citizens might just report too many false alarms. If you read through the comments, a couple of people had the same thought I had. Smoke alarms give false alarms very, very often, and if you have an alarm system the firetrucks are coming just in case. Don’t some smoke detectors measure radiation increases? Plus in the greater New York area you can’t sell a house without a Radon Test and in some houses there is a powered radon detector so you get a warning if the abatement system fails. So if I can’t have one of these. Can I use this?

(It’s an Alpha Radioscope, used for detecting alpha particles).

They can have my Geiger Counter when they pry it from my glowing hand – Link