I'm sort of embarrassed to be admitting this, but we "borrowed" the wireless link to someone in our neighborhood. You see, the cable company came out and cut off the wrong neighbors cable modem. They meant to cut off the next door neighbors because they were moving and went to our box instead -- Stupid people! So they apologized for the mistake, but couldn't get anyone out here for two days. Well, I had two EXTREMELY IMPORTANT emails I had to get out to people I work with, but had no way of doing it since we had no internet here. So, we thought, well let's drive over to Starbucks, but as we were driving honey said, go this way and drive slow (he's a network administrator for a big company), so I did and he said STOP! And he sent my emails out. I know it was wrong and I felt bad, but I was so happy that we were able to get my stuff out.
Anyway, my point is, it's actually really easy to get into wireless accounts if you haven't done the stuff Jawert said above. We literally did it by sitting on the road in front of the neighbors house. I know no one can get into mine because honey is a "securities expert" with this stuff. His company is actually involved in designing things for the military.
As for you being accountable, dtbmnec is right, the feds would look at the computer not the connection so you should be fine. In regards to deleting files, there are ways to get around it without all that expensive stuff that would make it so the files aren't recoverable.
Stupid People Have Stupid Children, Hence All The Ignorance In The World!