I’ve had a couple of readers IM me in the last hour to let me know that YPN ads are down/blank and have been so for ‘hours’. WebmasterWorld Forums also have a discussion on the topic of YPN Blank Ads as does Digital Points forum in the Yahoo Ads Down? thread which confirms that it’s wider spread than just a few scattered publishers.
Looks like it’s more than a quick glitch with some reporting up to 3 hours of no ads.
update: Some publishers are reporting that their ads are returning slowly to some of their sites.