There is a new extension to the Firefox browser called Firesheep.  You are not safe using unsecured wireless networks like you find in coffee shops.

"The Firesheep plugin can hijack your Facebook, Twitter, and Flikr sessions while you are connected to unsecured wifi. What do we mean hijack? We mean that it can steal your sessions, pretend to be you, and you won’t even know it."

Read more about it here.