I took the MarkerClusterer plugin for Google Maps (http://google-maps-utility-library-v3.googlecode.com/svn/trunk/markerclusterer/docs/reference.html) and enchanced its styling possibilities by adding support for the backgroundSize property (in case you want to use retina images).
You can find documentation for the script here: http://google-maps-utility-library-v3.googlecode.com/svn/trunk/markerclusterer/docs/examples.html
The only addition from me is the property 'backgroundSize' when adding a styles property in the options object.