This is the blogging equivalent of a retweet, but these articles say everything I would already say and have more to offer than I do:
How to Guard Yourself and Your Mac From Firesheep (Also applicable to Windows)
The bottom line is that a new Firefox add-in, Firesheep, allows other people to impersonate you over an unsecured wireless network. Some websites have taken steps to prevent this, but some, like Facebook, have not taken the steps to protect their users’ privacy from being violated.
As EFF states here, the real issue is protected WiFi, not public WiFi.
Be informed and safe on the internet, everybody.