Transform > 'Move center to vertices' preety much averages the possitions of vertices, so it depends on the topology,
A ball that has its top part subdivided a few times will have a center close to the top, while real center of mass should remain pretty mcuh in the same place.
I tried also rigid bodies where there is inertal properties > center of mass, but is litterally the same algorithm.
Here is a response that i found on google groups:
ThxTo find the center of gravity (or "centroid") of a polygonal mesh:
convert all of its faces to triangles, and average the centroids of
all of the triangles, weighted by the doubled area of each face.
Wikipedia calls it <a href="http://en.wikipedia.org/wiki/
Centroid#Centroid_by_geometric_decomposition">Centroid by Geometric
C = Centroid <vector>, A = (area of a face * 2)
R = face centroid = average of vertices making the face <vector>
C = [sum of all (A*R)] / [sum of all R]