Ryan Narain had an interesting blog earlier this week "Hacking with Metasploit on a Nokia N800". In it, he describes how for only $399 hackers can use a wireless exploit penetration testing tool, Metasploit, in a covert manner. This type of attack is relatively easy to do and can be done by someone walking around with a backpack and a Nokia Tablet PC inside it running Metasploit.
The result can be penetration of your wireless networks, the insertion of malware, the capture of identity authentication information and the resultant loss of enterprise information. Ensure your networks are properly defenced against this type of cheap and effective attacks.