Up until recently it took some Jedi skills to pull passwords and things off an unencrypted network, like the kind you find at your average coffeeshop, airport or buddy’s house.
Not anymore.
You’ll want to go to BoingBoing if you want the whole story, but the short story is a Firefox extension called Firesheep can capture passwords to many of the sites you use, no Jedi skills required. Your banking account is most probably secure, but sniffing out your Facebook account is quite simple.
To learn more: http://www.boingboing.net/2010/10/27/sheep.html