Reportedly, the ads displayed on publisher sites were loading extremely slowly or not loading at all. Here are select quotes from the threads.
My ads have been so poorly served today, about half the time they don't show up at all and the other half only 1 or 2 ads will show up when normally 4 would.
We're removing YPN and replacing it with AdSense now to salvage some revenue until (if/when?) YPN get its act together. A note in our account letting us know what is going on would be nice. Are listening, YPN?
First reports started coming in on Saturday at 8:45 in the morning (EST). It seems like the issues with the ads began to resolve themselves the day after at around 9pm (EST). This is based on the reports at the forums. I do not have official confirmation from Yahoo on this reported outage.