However, there are many commercial and free programs such as Kismet that can quickly decipher "hidden" SSIDs. Other programs such as Netstumbler, may not fully resolve the SSID, but they will show the existence of access points with a null SSID. Netstumbler does this by sending out active probe requests. Even if the SSID is hidden, per the IEEE standards, Access Points are required to respond to such requests. Although this response doesn't contain the actual SSID, it contains other useful information such as the MAC address, channel number, signal level, etc. Attackers can use this information as a spring board for their attack, just as they could if they found out the actual SSID.
Here's the kicker: legitimate users also need to know the SSID in order to associate to the wireless network. Hiding the broadcast of the SSID often causes legitimate users confusion about where to connect, leading to a dramatic increase in calls to the organization's help desk.
Bottom line, since the SSID can be detected so easily, it provides little security from malicious attack. It is more of an inconvenience to your own users than it is an obstacle to even semi-skilled hackers.
Tomorrow is the conclusion to this series, which discusses time slicing wireless intrusion detection.