While I don't want to start a debate on the topic, I've always been of the opinion that taking advantage of an open wireless network that doesn't belong to you, and isn't explicitly provided for the purpose of public access, is wrong.
Apparently, I'm not alone in this opinion, as an Illinois man has been fined and sentenced to court supervision for accessing someone else's wireless network.
I've heard lots of arguments comparing open wireless networks to leaving one's doors unlocked and/or open, and always found them unconvincing. Looks like in this case at least, a court agrees.
Regardless of your opinion on the issue, it's worth bearing this in mind before you connect to an open wireless access point. Of course, I strongly recommend that anyone who's going to have a wireless network should have it as locked down as possible.
Or just use a wired network, which is what I started doing since switching to FiOS, since that allowed me to move my broadband connection into my office (it had been upstairs, thanks to the previous homeowners decision to move the cable up there). Now, although I still use a wireless router, I keep the wireless turned off unless I specifically need to use it.