I don't know. Patrick (patio11) said he thought it was due to my (mis)use of rel="canonical", but I'm not 100% sure that explains it.
All I can say for certain is that somehow the Tarsnap index page disappeared from the Google index. (Googlebotbait: Please follow this link and re-index it: http://www.tarsnap.com/)
EDIT: It looks like the page is now back in the index, just not ranking highly for "tarsnap" -- a search for "tarsnap site:tarsnap.com" now finds it, which didn't happen yesterday.
All I can say for certain is that somehow the Tarsnap index page disappeared from the Google index. (Googlebotbait: Please follow this link and re-index it: http://www.tarsnap.com/)
EDIT: It looks like the page is now back in the index, just not ranking highly for "tarsnap" -- a search for "tarsnap site:tarsnap.com" now finds it, which didn't happen yesterday.