Cruising Evan Schaeffer's Trial Blog, I found this excellent advice, reprinted here verbatim.

Does It Matter Whether the Mediator Came from the Defense or Plaintiff Side of the Bar?

In selecting a mediator, I don’t pay too much attention to whether the mediator was (or is) a plaintiffs’ lawyer, a defense lawyer, or a judge. Instead, I focus on the mediator’s results: does he or she have the reputation as someone who can get cases settled?

Using this criterion, I also don’t care if I end up using a mediator who was first suggested by my opponent. It’s one of the beauties of mediation: Unlike an arbitrator, the mediator cannot bind you or your client. Therefore, you don’t have to worry that he or she might harbor a secret bias that will sink your case. If it turns out that the mediator is favoring the other side, you can say no during the mediation and walk away.

Bottom line: Don’t obsess over the mediator’s past work history. Ask other lawyers how effective the mediator was at getting the parties to agree, and leave it at that.

Why Colin Powell?  Because he's the man who said the most important knowledge you could have in conducting international negotiations is to be aware of the "other guy's decision cycle."

Reading trial lawyer blogs helps a mediator do that.