It really is amazing how many open wireless network you can find. If you are somewhat technical, get a wireless scanner (like NetStumbler) and see what you can find. Once you are in there, you can use an open source tool like Metasploit to attack, I mean test, the machines you find on the open network. Statistically, you'd probably be successful in compromising machines a majority of the times you try.
Yes, that's scary stuff. It's also why the first step on Security Mike's Guide is to secure your networks. One of the common misconceptions is that you need to stop broadcasting your SSID, which is the network identifier of your wireless network. I'm with Steve Riley on this one. He does a pretty good treatment about why it doesn't matter whether you broadcast or not.
Whether someone can see your network or not is besides the point. The real question is whether they can access it. By doing some very simple security configurations on your wireless router, you can make it a LOT harder to penetrate.
Photo credit: dasmart