What do you mean where? Also you should learn to do it in the command line using tools like hamster and ferret, firesheep is pretty buggy in my opinion.
Thanks - that's helpful. I was messing around with it today... but since you guys recommend Hamster and Ferret, that's what I'll learn next.
By "where" I was just wondering if anyone on this board had ever gone to some public wifi or place of business and tested it out (with permission, of course ;-).