If someone does something illegal using your wireless internet, you're responsible for it. You can't just leave your network open to have an excuse for anything illegal done on your connection.

Also, it's been reported numerous times that connecting to a wireless network that isn't secure is very dangerous. I was reading recently about some hackers convention in Las Vegas where a guy was demonstrating what could be done. He stole some dude's Gmail account right there, and the guy whose e-mail account was stolen was in the audience on the public wireless network they had there.