Yahoo and many Yahoo sponsored sites, like Flickr, experienced some very wide-spread downtime that many users reported. Some suspected denial of service attacks or compromised servers, but the actual fault seems to be much less serious than that.
In a statement made by Yahoo, they indicated that there were issues with their backhaul providers that provide them with network service:
"For a brief period early this morning, some Yahoo users may have experienced delays in accessing Yahoo services in the U.S. due to network carrier-related issues. The problem has since been identified and corrected by our network vendor. We know that this may have caused some inconvenience, and we apologize to our users who might have been affected."
Actual details weren't provided, but it seems they wanted to make sure that their users understood it wasn't a security issue. Regardless, all their sites are back up and it seems no foul play was afoot.