Caveat: This discussion relates to Halacha independently of the "Law of the Land." As Dina d'Machusa Dina, anything that we write must be reexamined in the light of any existing or eventual legislation.
Halacha does not recognize rights that are intangible. Radio, television and other wireless transmissions that cannot be seen, felt or otherwise sensed, and that do not remain in the domain in which they were generated, are not subject to ownership. (In Meseches Bava Basra, in cases in which a field is sold without explicit access, the Gemara cites an opinion that the new owner must either rent access or "fly through the air" to his field. Clearly flying over another's property is not impinging on it in any way. It follows that I can make use of anything that travels through the air, even through your property - such as wireless waves.)
Although it is possible that by "borrowing" bandwith one slows down the owner's connection, this is not an Halachic damage, as it falls into the category of Hezek she'Eino Nikkar (see the fifth perek of Gittin) - damage that is not apparent, for which there is no liability. Moreover, although one who causes his friend a tangible loss is ethically obligated ("B'yidei Shomayim") to make restitution, even when the damage is not apparent (e.g., if one renders another's food inedible by making it Halachically forbidden in a way that is not apparent, etc.), that is because a value can be placed on the loss, for although the damage is not apparent, the damaged items are apparent, tangible, and their loss of value is objective and definite.
Therefore, in a case in which there is no encryption or key, one may make of any wireless detected by one's computer.