I've created a new page in the Plazes Google Code wiki to gather operating system-specific ways of obtaining the MAC address of the
default gateway (which can then be fed to the Plazes API as a networks[mac_address] parameter to look up associated Plaze(s)):
http://code.google.com/p/plazes/wiki/FindingMACAddress
I welcome assistance from others in enhancing and fine-tuning the information there.