After a hard few months, certain database shards caught quite an illness today. But they are on the mend presently and the site will either be back up shortly and experiencing a few hiccups into the wee hours, or the it will be back up in about an hour, completely better.
We know there is no better Saturday night (or various parts of Sunday, as it may be for you around the globe) than spending time with Flickr, and we are working away in earnest to have a freshly scrubbed and speedy fast service for you.
UPDATE: as of 11:29pm pacific (GMT-8), we estimate that the site will be back up in just under an hour.