Hi Gabriel,
The mapping on the frontend is handled by media/js/ushahidi.js .. which wraps a lot of openlayers functionality.
On the main page, you should be able to update the map focus by doing something like this (in js):
map.updateMapCenter({latitude: 0, longitude:0});
The quick hacky way to build your cities switching would be to hard code the list of cities and latitude into the frontend view, add .click() handlers to each link to a city, make the click handlers call map.updateMapCenter..