It's called Active-Trac ID.
Basically it is a low powered scanner that picks up a cell phones sim/pim and displays the phone number on the scanner.
People in NYC have gotten their hands on some greymarket units from Hong Kong and have been using them to get phone numbers of people, usually hot girls, at malls and various other places. Only drawback is if there are a lot of cell phones in the range of the scanner, it will grab them all, but you won't be able to single out whos number belongs to who.