Comment BT in the UK (Score 1, Informative) 203
BT already does this in the UK. By default, it's enabled for all customers, but you can disable it. it's called BT Fon. Basically BT customers who opt-in get to use internet through any other customer's wifi for free, other people can pay to use it otherwise.
http://www.btfon.com/
http://www.btfon.com/