Your rights to your own property are not contingent on how well you secure that property. Maybe your neighbour has a few outdoor power points, so what you should do is buy a few really long extension cables, jump the fence and plug them in, then reap the rewards of free electricity! It's his stupid fault for not putting a lock over the top of them!
Not the same thing at all, as it would be pretty impossible to do this accidentaly and you would be trespassing to get to it as well. The case may be different however if you set up an extension lead running off your property onto public land and left it mounted to a tree with a sign saying "Unsecured powerpoint!" then im sure you would not be able to blame or even sue someone for plugging something into it. (Or even if you were successful this would hardly be fair justice as you practically invited them in) Especially if their plug jumped into it automatically by itself.
Dont get me wrong, I believe 100% full-time leeching is moraly wrong, i just dont see how it can be illegal. I pay for my internet, i have a 100Mbit hardwire connection to the net and WPA security on my router which i allow my neighbour to use sometimes. However on the RARE occasion i am out with my laptop in a park or something, i get a signal and my laptop connects itself to a network then i will use it, simple as that. Im not nicking anything (no pay-per-meg here) and i certainly don't feel guilty for it. I believe if the host doesn't want me there they would close the door (as pointed out above ignorance is no defense).