One of the other forums I frequent has a ‘Members Map’ using Google maps which people add themselves too. I’m sure it would be very easy to set one up for PP as well, and once created perhaps add a link to it in the automated ‘Welcome’ email/message to encourage new members to add themselves. It does require people to sign up for a google account though (quick and free).
Here is the map for the other forum, as an example – http://maps.google.com/maps/ms?ie=UTF8&hl=en&msa=0&msid=214046924149280268662.0004a029b2cb533998cf8&ll=52.295042,-2.120361&spn=3.541123,14.501953&z=7