This is one of the more interesting uses of Google Maps I've seen. A group of Turkish activists, protesting their government's censorship of websites such as YouTube, and Last.fm, have organized a "virtual march" on Google Maps.
I think this concept could really take off here as well. What if instead of organizing a real March on Washington, an activist group organized something similar to this Turkish protest. Would it have the same impact? It certainly would be cheaper to organize and since the "cost" to any given protester of joining the march would be minimal, perhaps it would attract many more people. Or what about using something like this as an add-on feature to online petitions? Wouldn't it be effective, for instance, to see millions of pushpins on the map of the U.S. for all the people who have signed a certain petition?