Alfie:
It is indeed very illegal. Very common, but very illegal. Below is a CNN story about a man that accessed his laptop while parked in front of someones home, evidently not realizing he accessed their WiFi. His Mac address showed up on their router and boom. My son who is in a computer networking program said that it is essentially breaking and entering at least cybernetically.

http://money.cnn.com/2005/07/07/tech...reless_arrest/

God Bless....In HIS Service....Dan