The problem with many of the arguments above is that:
a) laws differ from state to state and country to country, and
b) people are getting confused between ethics and law
In my opinion, it is perfectly ethical to use an open AP as long as it does not disadvantage the owner (ie. don't use it when the owner is using it, small data usage, legal activities etc). If it does not negatively affect them, then it is not unethical.
If it is illegal in your area then it is illegal, no ethical justifications (including analogies) will change that.
Remember that ethics and law are not even nearly the same thing (some above have clearly been mistaken).
