Cluster Measure for geographical data

What you are looking for is the Haversine formula which calculates the distance between two points on a sphere. 

(See below for a simple explanation of the formula along with a javascript implementation)
http://www.movable-type.co.uk/scripts/latlong.html

At first glance, you'd need to do (as an example & assuming noone has done so already) one of the following. 

1-Change the algorithm for calculating the distance between the cluster points to use this formula in the DBscan operator. 
2-Install the R extension & use someone elses implementation http://pastebin.com/TL81ycx9
3-Calculate the distance between all points using a Groovy script version of Haversine & lots & lots of loops. http://svn.codehaus.org/grails-plugins/grails-solr/trunk/src/groovy/org/grails/solr/Haversine.groovy

Option 2 looks like the easiest, but that said, this is RapidMiner so it's possible that someone has already done this. 

Hope that helps!

Additionally I've found this that should help also, it just needs converted from PHP which shouldn't take too long. 

http://www.appelsiini.net/2008/11/introduction-to-marker-clustering-with-google-maps

Good luck!