Two people have been cautioned for using people’s wi-fi broadband internet connections without permission.
Neighbours in Redditch, Worcestershire, contacted police on Saturday after seeing a man inside a car using a laptop while parked outside a house.
He was arrested and cautioned. A woman was arrested in similar circumstances in the town earlier this month.
There are those who would argue that leaving your broadband connection open for others to share is harmless. Providing you have locked down access to machines on your internal network, what is wrong with giving away bandwidth that you are not using?
Some argue that drive-by users who enter illegal porn sites, or send massive amounts of spam would be hard to trace and that the owner of the network would held liable for any actions by these free loaders.
Wi-Fi Planet – Securing your Wi-Fi Connection
intelligentedu – 5 Simple Steps to Securing your Wi-Fi Network
Linux.com – Secure your Wi-Fi Traffic Using FOSS Utilities
From Wikipedia – Wi-Fi Social Concerns
Measures to deter unauthorized users include suppressing the AP’s service set identifier (SSID) broadcast, allowing only computers with known MAC addresses to join the network, and various encryption standards. Access points and computers using no encryption are vulnerable to eavesdropping by an attacker armed with packet sniffer software. If the eavesdropper has the ability to change his MAC address then he can potentially join the network by spoofing an authorised address.
WEP encryption can protect against casual snooping but may also produce a misguided sense of security since freely available tools such as AirSnort can quickly recover WEP encryption keys. Once it has seen 5-10 million encrypted packets, AirSnort will determine the encryption password in under a second. The newer Wi-Fi Protected Access (WPA) and IEEE 802.11i (WPA2) encryption standards do not have the serious weaknesses of WEP encryption, but require strong passphrases for full security.
Recreational exploration of other people’s access points has become known as wardriving, and the leaving of graffiti describing available services as warchalking. These activities may be illegal in certain jurisdictions, but existing legislation and case-law is often unclear.
However, it is also common for people to unintentionally use others’ Wi-Fi networks without explicit authorization. Operating systems such as Windows XP SP2 and Mac OS X automatically connect to an available wireless network, depending on the network configuration. A user who happens to start up a laptop in the vicinity of an access point may find the computer has joined the network without any visible indication. Moreover, a user intending to join one network may instead end up on another one if the latter’s signal is stronger. In combination with automatic discovery of other network resources (see DHCP and Zeroconf) this could possibly lead wireless users to send sensitive data to the wrong destination, as described by Chris Meadows in the February 2004 RISKS Digest. 
In Singapore, using another person’s Wi-Fi network is illegal under the Computer Misuse Act. A 17 year old has been arrested for simply tapping into his neighbor’s wireless Internet connection and faces up to 3 years’ imprisonment and a fine.