There actually seems to be a method in GMap3. It's a static method of the google.maps.geometry.spherical
namespace.
It takes as arguments two LatLng
objects and will utilize a default Earth radius of 6378137 meters, although the default radius can be overridden with a custom value if necessary.
Make sure you include:
<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false&v=3&libraries=geometry"></script>
in your head section.
The call will be:
google.maps.geometry.spherical.computeDistanceBetween (latLngA, latLngB);