Hiding an SSID will not hide a wireless network
"Hiding an SSID will not hide a wireless network, so ignore any such advice. By the way, also ignore any advice that says to use MAC address filtering. It's amazingly trivial to spoof the MAC address of an allowed supplicant -- simply sniff the traffic, look at the MAC addresses, and use the neat little SMAC utility to change your MAC to one that's permitted. Nonbroadcasting networks are not secure networks. The right way to secure a wireless network is to use protocols that are designed specifically to address wireless network threats. If you're still using WEP, either static or dynamic, I encourage you to move to WPA2 as soon as possible."
Read the entire article at http://blogs.technet.com/steriley/archive/2007/10/16/myth-vs-reality-wireless-ssids.aspx