I just got accepted to the YPN and it says to expect it to take two days for ads to show after implementing the code. If I'm going to take Google ads off one page to test Yahoo, I don't want to wait 2 days for ads to show! To the people who've already put up Yahoo ads - does it really take 2 days??!
I added mine last night and they didn't show for an hour. I went to bed and they were there this morning. Not right away, but not 2 days!
Yahoo also has a new site targeting feature, where you can tell it what ads to show. Just go to "Ad Setup" then choose "Ad Targeting".