You ARE using their resources when you grab an IP through DHCP. It takes processor power to register that ip.

If you really wanna stay in the clear, it's best to use a sniffer and pull things out of the air, as opposed to connecting to anyone's machine. They can always get you that way (legally).

Then you can make the argument about whether breaking someone's encryption (wep, or whatever) is illegal. Then you're not using their resources, but could be violating their intellectual property rights. But then who knows. Of course you can always argue that all your doing is passively pulling down the airwaves.

More to think about.