An excerpt from a article on Smartmoney:


"I will use your Wi-Fi -- and get you arrested."
Nearly one-third of Americans admit to using their neighbor's Internet service, nearly double the number from two years ago, according to a national survey by the non-profit Wifi Alliance Trade Alliance. Such thieving can push your data usage above its monthly limit and increase your Wi-Fi bill, says McCall Butler, a spokeswoman for AT&T, who recommends that customers protect their Wi-Fi network with a password and change it regularly. Worse, there's no controlling what Wi-Fi thieves do with your signal, and if what they're doing is illegal, you could be in hot water.
Barry Covert, a lawyer based in Buffalo, N.Y., is currently representing two clients -- one in Buffalo, N.Y. and one in Milford, Mass. -- who he says had their wireless internet hijacked by neighbors to download child pornography. The clients are no longer facing charges -- the U.S. Attorney's Office and Immigration and Customs Enforcement, a division of the Department of Homeland Security, issued an official apology in March to the family in Buffalo, and the Federal Bureau of Investigations told SmartMoney.com that the agency believed the people in Milford were innocent. Neither case went to court, but if they had, Covert says legal fees could have run to $100,000.
The solution is simple: Secure your Wi-Fi, and change the password regularly. It's not failsafe, but it sets up an obstacle, and that can be enough to encourage a thief to move on to the house down the block. "If you use technology, you need to know how it can be used against you," Covert says.

Or at least spoof your mac address and or buy your Alpha adapter- Amazon.com: Alfa AWUS036H 1000mW 1W 802.11b/g USB Wireless WiFi Adapter With Original Alfa Screw-On Swivel 9dBi Rubber Antenna and Suction cup Window Mount dock: Electronics off of craigslist in a different city, so your credit card card won't be associated with it. And don't park your van right outside their house.