I created an extension to serve up Yahoo Publisher Network ads based on the existing code for the Google AdSense extension. I haven't decided which is better, YPN or AdSense, but word is they are pretty close.
i thought about making a yahoo version myself (i've switched any of my sites that used google before to yahoo for now), but didn't think there was actually a wide enough audience to use it.
you're probably right, not a wide enough audience at this point since they are in Beta and limiting the amount of publishers, however I expect this to change soon as YPN picks up steam.
I figure if 5 people get some use out of it, then it was worth it :)