I found this on slashdot in one of the comment sections, arguing that it should not be illegal to access open access points. I rather like it...
Imagine your neighbor has a TV going loudly - he has cable TV and you do not. You hear a show playing you've been meaning to see. You yell over the fence, "mind if I come over and watch that show?" The neighbor's butler yells back, "Sure, come around through the gate.". You go over, sit down and enjoy the show. After the show is over, the owner shows up, and is PISSED because you are there.
The neighbor has not lost any property, but has been denied payment for a service he has performed. (providing you with entertainment) Unfortunately for the neighbor, you were allowed free access to the entertainment indirectly by the neighbor. The neighbor has no legal grounds against you because you were acting with permission of an agent of the neighbor. (the butler)
This is very similar to the issue of open access points. The wireless router being the butler that's been told to allow anyone that asks to be given free internet access. Just because you get upset that the currentl policy of your own access point bothers you does not give you free license to sue someone that has taken advantage of your offerings.