It is very easy to mix up analogies when discussing this topic. Some are completely wrong, some are close to the mark.
@The first comment, comparing it to leaving your house/car unlocked: This is actually quite close. Its is still a crime if someone nicks the car/your telly. Its your own stupid fault, but it's still a crime and the theif (if caught, something the police aren't so good at... whats their job description again?) would be prosecuted. Your insurance would not pay out because you were a dumb f**k, but a crime has still been committed.
Same applies to wifi: Leave your router open, and you desrve to have it piggybacked. Leave it open, and leave unsecured shares on your network, with files containing all your personal details etc... you deserve to have your identity stolen, your important documents deleted etc.
However I also agree with Matthew Robinson' comment: The DHCP server was asked for an address, it granted it. It sould be argued that this constitutes an agreement that you may use the network. If a gateway is included, it could be argued you are being invited to use their internet connection. I doubt a court would see it that way.
Bottom line: If you don't want people on your wifi, don't leave it unsecured, and don't use DHCP without MAC filtering. If you're stupid enough to leave the door to your house open and a note on the dorr telling people where all your decent stuff is (equivalent of open wifi and open DHCP) you are an eejit who desrves all he gets.