I often wondered whether it was legal to do so, having had neighbours hijack
my connection, and having discovered that my own kids were sometimes
inadvertently hijacking a neighbour's connection.

Yet another way for law abiding citizens to find themselves in breach of the
law, then:

http://news.bbc.co.uk/1/hi/technology/4721723.stm
[quote]
A recent court case, which saw a West London man fined £500 and sentenced to
12 months' conditional discharge for hijacking a wireless broadband
connection, has implications for almost every user of wi-fi networks. It is
believed to be the first case of its kind in the UK, but with an estimated
one million wi-fi users around the country, it is unlikely to be the last.
"There are a lot of implications and this could open the floodgates to many
more such cases," said Phil Cracknell, chief technology officer of security
firm NetSurity. Details in this particular case are sketchy although it is
known that Gregory Straszkiewicz had "piggybacked" on a wireless broadband
network of a local Ealing resident, using a laptop while sitting in his car.