I found this excellent article by Mike Elgan over at Computerworld.com that details two reasons why it is not unethical, nor should it be illegal, to use someone else's open WiFi for your own Internet activities.

Elgan makes two excellent points. First, he states that by using a WiFi network, you are asking for, and receiving, permission from the owner. Second, your computer cannot be on their network unless their network is in your computer, which is something over which you have no control, since WiFi is is a radio signal broadcast to the (local) world.

I agree with him that the laws regarding "theft" of WiFi should be changed (or done away with altogether), especially given that many devices connect to open networks automatically when they are within range of the WiFi signals those networks broadcast. If users do not wish to have others connecting to their WiFi networks, simply disallow it. Failure to do so is implicitly granting permission to use the network.

» Read the article