Boxtops:
If you use a neighbour's wi-fi, you'll be adding to the amount that 'they' download. If they've got an account with no restrictions on downloads it obviously won't cost them anything, whereas someone who has a limit on his downloads might have to pay extra (or even have his internet access cut-off or restricted if his account goes well over the agreed limit).
Either way, it's still a criminal offence to use someone else's connection without their permission.
Chris