Just for clarity, they need the
Wireless MAC address of your access point. The wired (ethernet) side has a MAC address too.
With a Mac laptop connected to an Airport, just Option-click the wireless icon at the top right of the screen. the MAC address is made up of eight 2-digit numbers separated by colons and is labeled BSSID, at least on Snow Leopard.
Skyhook warns that it might take weeks for the update to take effect, based on how often their clients update the data.
FWIW, I used my AirPort at my old office, about 12 miles from here and NEVER registered the MAC address with anyone; also I have my iPod Touch connected to a different AP on the same network, but still the iPod reports my location as the old office despite having moved 4 months ago!
Walt