Fact number 1: when you log on to a public Wi-Fi hotspot, you are broadcasting your login name and password (and sometimes also a credit card number) over open airwaves.
Fact number 2: about 90% of mobile devices lack intrusion-detection devices that would impede to transmit the network they are part of to hackers.

As public Wi-Fi access points are mushrooming and companies are willing to take advantage of mobile technology, the risks of wireless hacking attacks and viruses (called by security experts “wireless transmitted diseases”) is becoming worrying.

Read article in USA Today and their feature on Wi-Fi security.