Hey,
I recently tried using the Maps module with clustering functionality. Apparently this causes the module to include a reference to an external javascript located at this CDN: http://google-maps-utility-library-v3.googlecode.com/svn/trunk/markerclustererplus/src/markerclusterer.js - which isn't there. This caused the module to break or at least disregarding any setup concerning clusters.
It's perhaps a little dangerous to rely on a CDN when it's beyond template control. Any page with the maps module using clustering will have an error going forward. However, we can include an alternative file to restore functionality. I spent some time looking for a replacement that didn't conflict with the other scripts associated with the module and found this one: https://code.google.com/p/google-maps-utility-library-v3/source/browse/tags/markerclustererplus/2.1.2/markerclustererplus/src/markerclusterer_packed.js?r=473
Perhaps the module should be updated and contain the dependencies locally rather than relying on CDNs that may change.
//Martin