Here in the UK, there have been several well publicised cases where Police have been called to investigate people loitering in cars, and just in the street. Subsequently these people have been prosecuted for stealing bandwidth. So in the UK at least jumping onto an unsecured wifi network without permission is clearly defined as theft.

your tablet connecting to the wifi, and downloading a few hundred KB of tiles for maemomapper would probably go unnoticed, but any more visible than that, and you would be in bother.

If someone gave permission, by naming their connection "please use" then that would be OK. Leaving it at the factory default is not granting permission to use.