One of the dark attractions at DefCon is the "Wall of Sheep". The idea is simple - a bunch of people with sniffers sitting together capturing all the unencrypted traffic that flies through the air via the free WiFi connections available at the convention.
HTTP, POP3, FTP and ICQ logins are definitely intercepted, and others may be.
I was quite suprised to see lots of Myspace accounts listed on the Wall of Sheep. It turns out that unlike other community services such as Orkut or LinkedIn, Myspace's login is totally unencrypted and prone to sniffing.
So if you are a MySpace user, I suggest you stay away from your account next time you connect to the Internet by a public WiFi network.