During our visit to the Virus Bulletin conference 2008 last week a man was arrested in Belgium for using someone else's unsecured Wifi connection to get on the Internet. (More details in Dutch available here).
The case is interesting because the only thing this guy did was use the connection to get onto the Internet - what we call Wifi "piggybacking," or logging on to someone's open 802.11b/g/n network without their knowledge or permission. And quite a lot fo countries (such as the UK and Belgium) have laws making this illegal.
There've been other cases like this in the past: Illinois man was arrested and fined an $250 in 2006 for using an open network without permission, while a Michigan man who parked his car in front of a café and snarfed its free Wifi was charged back in May 2007 with "Fraudulent access to computers, computer systems, and computer networks." http://www.viruslist.com/en/weblog?weblogid=208187581