There are different laws in different countries. But for my part I don't see any moral, ethical or criminal problem with connecting and syncing mail for 30 seconds or so, disconnect and go away. I certainly wouldn't mind anyone using my own network that way. And wi-fi devices can be set up to automatically connect to open networks, the user won't even know what the essid is (e.g. if it's 'default', 'linksys', 'homenet', 'private-but-open-stay-off', 'myfreenet-for-light-use' or whatever).

What I never do though is abusing someone else's network (I don't download huge amounts of data or surf for long periods etc.). I sync email mostly. What is legal or not legal in my country I'm not sure about, but I used to walk to the busstop, go online for 30 secs and synch avantgo on my Palm, then walk on the bus. Because there was a reachable network there. Legal? I think so, but I'm not sure. Feels legal? Yes. Would I mind if I were the network owner? No.

In any case, whatever else, I think the age-old medical oath should be followed: "First, do no harm."