From the Stumbler to Plazes Web Service Test page you can upload the logfile from a wifi-scanning application like MacStumbler or netstumbler and get XML returned that contains suggested Plazes for every MAC address you encountered.

This is a quick hack, not an official Plazes product, and is bound to disappear without notice.

By way of testing this, I took a walk around the block here at the office, and found, to my surprise, 78 wireless networks in the immediate neighbourhood. Only 3 of them were associated with Plazes.