Last time I went to Toronto with my friend and we discovered that there are so many open wireless access point in downtown Toronto. I am glad that people were such generous that they willing to share bandwidth with others; however, open WAP may post serious security issue.
Why I saying this is because WAP acts like a hub in the network, anyone who put a "packet sniffer" onto the wire can monitor all traffic going in and out. The result is everything you do will be transparent.
I believe that it is necessary to encrypt our wireless network once we set the network up using WEP or WPA. But what if in the airport or coffee shop, where wireless connection is available but not encrypted? One approach is to turn on your local firewall. And possibly to use SSL or VPN.