It is interesting that you ask about Yahoo. It is generally understood that when you search for back links in Yahoo, that Y lists them in order of “importance” or weight.
My number one back link on Yahoo is from Matt Cutt’s. I posted in his blog with my url. And he is using the no follow tag for blog comments.
So, it would appear, that Yahoo is not recognizing the no follow tag.
[edited by: kamikaze_Optimizer at 5:43 pm (utc) on Feb. 14, 2008]