This question comes up from time to time.

In the UK the answer is YES, obtaining free Internet access is an offence under the Communications Act 2003 and a potential breach of the Computer Misuse Act.

Only yesterday a man was arrested in Chiswick for casually accessing the unsecured wireless network of a near by house. 

Link: http://www.guardian.co.uk/uk_news/story/0,,2154284,00.html

If you have a wireless connection in your home or small office and that connection is unsecured There is a good chance that other people will be able to see your connection and be able to connect to your network.

They would do this to access your Internet connection but in connecting to your network they could also have access to all the computers on the inside of your wireless network. 

Tech Tip: So find the manual that came with your wireless device and take the time to learn about wireless security. Even the relatively insecure WEP64 system which is the easiest to setup will keep out everyone apart from those who are determined to compromise your security.