Sophronisba - 11:13 pm on May 13, 2013 (gmt 0)
Well, I still don't know what the problem is. It did hit all areas of our site pretty uniformly; I wasn't able to isolate any one factor or keyword or anything like that.
We did recreate our sitemap, and I also deindexed many low-traffic pages in case that was hurting us somehow. Today is looking better than yesterday, and this afternoon is looking better than this morning, so that seems to be progress.