It's pretty black and white.  If you are using someone else's wireless connection (whether secure or insecure) without their knowledge and permission, than you are in the wrong both morally and, in most places, legally as well.  Even with the owner's permission you are still not in the clear.  Many large service providers, like Comcast, explicitly prohibit any sharing of the network connection in their terms of service.  Most service providers also do have bandwidth caps or variable cost plans.  Throw in liability concerns, risk of infections or data theft, etc., and it's just a bad idea.