Don't you guys read the news? There was a guy in Florida arrested over this very thing back in July. The network was unprotected and he was only using it for casual surfing.
Guess what? Third degree felony! That's right dumb dumbs...it's illegal.
However, with that said, it all comes down to interpretation of laws. The association with the access point through the air is not a crime. The airwaves are owned by the government, not the homeowner. If you're using equipment to pick up the signal then no crime is committed.
What they would get you on if they were going to press charges is the unauthorized use of the ISP network, assuming you weren't doing any illegal activities on top of that.
The case in Florida all boiled down to the mans intent. Pay attention. That's the key word. If you have no intent on harming the network or abusing its traffic in any way then you'll probably have no worries. However, depending on who's network, what type of business they're in or how wealthy the individual or organization is, you could be looking at a world of trouble. At the very least it may cost you big monetarily just for court fees.
Bottom line? Don't do it if you don't want to get in trouble. Chances are you'd never been caught or charged with the crime, but I'd hate to seen a fellow AT member in that kind of trouble.