Most people by now have heard of sidejacking. That's where a bad person (and a laptop) connect to one of the unencrypted wireless access points at school, and listen for the cookies of other students who are also connected and browsing the web. It's trivial to accomplish, and there is no easy way to detect it. It has been publicized as being the method used to hack your facebook or email. This hack can easily lead to partial or full identity theft.