On the PHP::Impact blog Federico Cargnelutti points out a PHP library that lets you do clustering on Google Maps. What's clustering?
As maps get busier, marker clustering is likely to be needed. Marker clustering is a technique by which several points of interest can be represented by a single icon when they’re close to one another.
The tool, created by Mike Tuupola, lets you break up the map into cells and place points inside them. When there's more than one point in a general area, an extra icon is created to reference those points together (cleaning up the map and preventing you from having lot of little Google Maps markers cluttering up your map. Mike also has a blog post about the library showing it in a bit more detail.