It all depends on the situation. When it comes down to it, it IS illegal it just matters how far the person you are leeching off of wants to take it. If someone sue's you or you get caught by comcast "scanners" you can expect at the very least a fine. It's very rare that someone will notice you on their network. If it's unencrypted chances are they aren't smart enough to notice you on there, unless your sucking up the bandwidth and they start to get curious, then get friends or comcast to check it out. Worse case scenario
http://money.cnn.com/2005/07/07/technology/personaltech/wireless_arrest/
But, say you do get caught somehow (excluding sitting out in front of someone's house). If you atleast have some sort of defense you should get out ok as long as your nice about it all, and/or play dumb. It's not that big of a deal anyway unless you are hogging their network bandwidth, and then it really just comes down to personal morals. If you wanted you could just mask your computers identification info and they couldn't find out who you are without an extreme amount of effort.
Personally, I believe if you leech all their bandwidth by downloading things and then causing them problems then that is immoral. If you're simply browsing the net, doing the e-mail thing, etc... then obviously that doesn't affect them at all, and I don't believe to be immoral. Typically, a wireless connection has a 1.5mb connection and with you on there it won't make a difference. Unless, like I said, you are taking up alot bandwidth by doing multiple things.
I'm sure all of you speed atleast 1 mph over the speed limit, have looked at porn before you were 18, tresspassed on someones property, egged someones house (or tp'd), and half of us have smoked pot at one time or another... I can go on, but point is it's all the same in terms of legallity. Personally, I'm against smoking pot unless it's for medical use, and excessive speeding, but some believe otherwise just like those same people may be against wireless "Piggy Backing".