It's easy enough to pick up your neighbor's computer and mobile phone signals, I've been told. Dunno. Here are some articles found with Google. I am not able to critically evaluate these.

How to pick up your neighbors' Wireless Networks For Free, Article by Yahoo.
How to spy on your neighbors with a USB TV Tuner. Tom's Guide
(Note- these gadgets are cheap.)
Six reasons why you should secure your Wi-Fi Network, KSChang.
(Others say total security is impossible)
Piggy-backing (Internet access). Wikipedia
Long-range antennas- some for sale, but easy to make.
Articles on detecting a neighbor's use of your signals.
How to build a long range Laser spy system for eavesdropping on your Neighbors.
Residential Eavesdropping- Protective Countermeasures. (Many articles)

Tapping Fibre-optic cables. (Much information. www.blackhat.com has information on threats and defense.