Piggybacking is illegal

Above is an image of the spread and density of WiFi networks in a 1km square area centred around Deptford Church Street in Deptford, south-east London. It was made by warwalking the entire area, using a laptop computer (with an Orinoco Gold card and external antenna) running the Netstumbler software. 553 nodes were discovered, and about a third were open. Open WiFi nodes create opportunities for supporting public use of space and technologies. So it seems counter-productive to read this BBC article.

According to this report, surfing an open wifi node, or piggybacking, is illegal.

"Gaining unauthorised access to someone else's network is an offence and people have to take responsibility for their actions. Some people might argue that taking a joy-ride in someone else's car is not an offence either," Simon Janes, a former head of the Computer Crime Unit and now operations manager for computer forensics firm Ibas

This also seems to be the case in the US according to this article.