Full article here
SSIDs are useless
The 802.11 standard specifies the SSID (service set identifier) as a form of password for a user's radio NIC to join a particular wireless LAN. 802.11 requires that the user's radio NIC have the same SSID as the access point have to enable association and communications with other devices. In fact, the SSID is the only "security" mechanism that the access point requires to enable association in the absence of activating optional security features.
The use of SSIDs is a fairly weak form of security, however, because most access points broadcast the SSID multiple times per second within the body of each beacon frame. A hacker can easily use an 802.11 analysis tool (e.g., AirMagnet, Netstumbler, or AiroPeek) to identify the SSID. In addition, Windows XP does a great job of "sniffing" the SSID in use by the network and automatically configuring the radio NIC within the end user device.
Some network administrators turn off SSID broadcasting (which deletes the SSID from the beacon frames), but a hacker can still sniff the SSID from frames that stations use when associating with an access point. They just have to wait until someone associates or re-assoicates (e.g., when roaming) with the network.
Aside from sniffing the SSID, many wireless LAN administrators make it even easier by using the vendor's default SSIDs, which are pretty well known. For example Cisco uses tsunami and most other vendors use the name of their company as the default SSID. Just do some war driving, and you'll see that this is true.