Generally speaking, if your wireless connection (meaning the IP your ISP gives you) is used for something illegal, the rap is on you. If you're tech savvy, there are a handful of ways of tracking who uses your wireless connection, but I doubt you would be able to use "open wireless" as a legal excuse. I *think* some woman wound up being found guilty after someone used her wireless access to download mp3s.
If you have open wireless, use MAC filtering and turn logging on.
If you're looking for a place to download kiddy porn and consort with your al qaeda buddies, a neighbors wireless connection will do just fine, so long as you don't login to your gmail/myspace/bank account while you're on it.