I agree with Walter when he says that…
If I were the defendant’s lawyer, I would argue that (assuming its unencrypted) making a wireless network available to the public is tacit permission for others to use the network. With permission there is no unauthorized use, and no criminal activity.
Although sorry Walt, I’m not quite buying your alternative argument about “squatter rights” or “adverse possession”. A Wi-Fi network doesn’t suffer quite the scarcity as land (it’s orders of magnitude easier to create a new network than it is to create land).
Besides, no matter how long someone has been leeching off an unencrypted network, I feel the owner of the network always has to right to switch on encryption at any time. In a manner, it is similar to identity theft. I don’t care how long a thief has been using a victim’s identity, that thief should never have squatter’s rights or the right to attempt adverse possession on the identity.