Clustering of marker in the plugin GoogleMaps is possible with the kmlrenderer GeoXML (version 2.12o):

{mosmap kml='http://tech.reumer.net/images/batchgeo2.kml'|kmlrenderer='geoxml'|
maxcluster='25'|gridsize='7'|minmarkerscluster='3'|maxlinesinfocluster='5'|
centerlat='0'|centerlon='0'|zoom='1'|icon='http://tech.reumer.net/images/demo.png'}

• maxcluster - maximum number of markers on a page (default 650).
• gridsize - size of the grid of a cluster (default 15).
• minmarkerscluster - minimum number of markers in a cluster (default 5).
• maxlinesinfocluster - maximum of line of markers in a cluster in a inforwindow of a cluster (default 15).
• clusterinfowindow - method of opening the infowindow on a cluster, default click. This can be:
  • click
  • dblclick
  • mouseover
  • mousedown
• clusterzoom - method on the marker to zoom into a cluster, default dblclick. Same values possible as above.
• clustermarkerzoom - zoom level for a marker in a cluster shown as a link in the infowindow of the cluster, default 16.

You can change the clustericon by using the custom (animated) icon option.