If I get this right the question of having
one or many forums is wether you want one global map to represent all locations to discuss worldwide or if you have many sites all over the planet displaying only sectorial maps which then link to their own forum each. I any case its one forum per map.
Or looking at it the other way around - what Team seems to suggest - the clue would be to have one global map that allows you to navigate to
different forums. That is a great idea - however it is more than I was hoping to suggest here.
To illustrate the whole idea of "map-BB" have a look at openforum. They build
wikis that relate to various cities. The
open guide to Boston is leading in developing a map that represents their wiki-entries - though not (yet?) allowing for annotating locations via the map (see the
discussion I had with developper Christopher Schmidt).