There is a little post on getting Google Earth and Rails apps talking to each other at: GE-Rails. There is even a hint on using merb instead or Rails

Alessandro Pasotti has authored a utility to output mapserver layers as KML for display on Google Earth. This is written in PHP mapscript so will require mapscript to be installed with your standard Mapserver installation. Check it out here.

Although its a useful tool, it would be nice to have kml support in mapserver (C++) out of the box. Apparently expected is the release of libkml in 2008. So mapserver may have to wait for google to release libkml before adding native C++ support, provided the licensing is favorable.