A friend of mine use to have a scanner (they're legal because people use them at car racing events to hear what the drivers are saying in the pit). One night, we sat out on his balcony with his scanner and started picking up different portable phone conversations from around his neighborhood. Interesting stuff. Especially when we started picking up a conversation where someone was having phone sex.
It was even grosser when we realized that it was my friend's roommate!
Scanners are actually quite popular. Some people make a hobby out of sitting out of their porch and picking up portable phone conversations (which why they say to never give your credit card number over a portable phone).