When I flipped the switch on the 19th to point to the new site, I forgot to update my robots.txt. Of course I didn't want people finding the new one until it was done, so I had blocked everything.

I now know that PageRank can last for up to 5 days after you stop letting them crawl your site. On the 24th, search traffic dropped off a cliff. (What a surprise.) Now I get to learn how long it takes for PR to come back.