Let's see what the Feds actually have to say about this:
Unless I read it wrong (please feel free to check), the current revision of Title 18 says that you must intentionally access a protected computer without authorization, and as a result of such conduct, cause damage of $5000 or more in a 1-year period for this to be a crime.
So, let's say we just want to do some Internet surfing via a wide-open, unencrypted AP of unknown origin (probably a neighbor), from the luxury of the living room
-Is it intentional? YES.
-Is it a "computer" I'm accessing? According to USC 18 - YES. An AP is a computer by definition.
-Is it a protected computer? According to the definition in USC 18 - Not inherently. But in this case we'll be getting on the Internet, so YES.
-Is the access authorized? It's kind of subjective, but I don't believe so.
-Am I causing a loss of $5000 or more? Easily - See below.
"Loss" can include things like time spent responding to the incident, damage assessment, etc. So, "loss" can add up pretty quickly with these intangibles. However, it doesn't count unless the loss was incurred as a result of a crime.
So, to sum up - Chilling in your living room and surfing via your neighbors AP (as harmless as it seems) is a federal crime. Also, some states like Alaska now have laws that specifically prohibit doing this as well (Theft of Service).
When I first started checking this out, I figured there would be some loophole that makes it all "okay". Well, there still might be one, but I haven't found it yet. The only solace to the determined I can give is this - If you can make any one of the five requirements above a "No", then you should be in the clear as far as the Federales are concerned.
I tried to make this post informative and factual. Hope it helps those who are hazy on the subject.