From a few responses I got to my previous email I understand that I
did not formulate my question clear enough.

What I meant by "compare two clusterings" is not to find out which
clustering is better - this information is usually provided by the k-means
algorithm in a form of within-cluster sum of squares.

What I actually meant was a measure of how much these two clusters
are similar. In other words - if two points belong to the same cluster
in both clusterings this means that they are similar.

Thanks a lot....