I stumbled across this new plugin today that will make you forever question the safety of public wifi: Firesheep.
Without going through and explaining it from a technical viewpoint, from a very simple (and frightening) point-of-view people can easily log into unsecure profiles (like Facebook) simply by downloading this plugin.
I am not sharing this link with people so that they can spy on others, but so that people can be aware that this type of program exists. Be careful!
UPDATE: My friend Gary just shared with me this article on How to Block Firesheep Using BlackSheep.