Very interesting. I'd like to see this feature as well.
Feature Request: "Opponents in common"

where would you display this?
How about a link on the profile page right below the link that says: My Games Against:
Display it the same way as "my games against" does it.. In a grid on a new page.

Here's a related idea ... who-is-player-x?

You know.. I'm working on a chess.comm rss feed -reading app that could figure this out.
Chess Calendar:
http://code.google.com/p/chesscalendar/downloads/list
But I was just thinking, I could also have it ping each of your updating opponents feed once (or more times) a day and show who is in common..
When you start a game with someone, it would be neat to see a list of opponents that you and that player have in common, similar to facebook's "friends in common".
I figure its an easy feature to create with some value.