This may be a little old now, but for normal users its worth knowing that if you use public wifi (eg that other people you don’t know use), then you could be in all sorts of bother if you also use facebook or twitter or for that matter any site that requires a login and delivers permanent cookies to the PC.
http://en.wikipedia.org/wiki/Firesheep
I picked this up from Steve Gibson’s excellent podcast on security for Twit.tv
Mrben09 