diberry - 3:53 pm on Aug 28, 2012 (gmt 0)
I've been thinking more about 1script's remarks, and realized, duh, I did make one tech change around the 23rd.
Basically, I had changed my domain name months ago for branding purposes from OldName.com to NewName.com and 301'd everything from OldName. Around the 23rd, I switched OldName to a reseller hosting package, since all it's doing now is redirecting inbound links to OldName that I can't get people to update. There was no downtime for either site, because I left OldName's old hosting up for four days to cover the propagation period and then some. I've changed hosts on my main sites many times in the past and NEVER had a Google traffic drop like this, so it's hard to believe changing hosts on the redirected site would have such an impact.
If that's what's going on, then it may recover on its own. Or not - whatever G's priorities are, I know they're not my site health! :)
Fortunately, I have other traffic sources, so I'll get through this eventually. I would still like to understand what on earth caused Penguin to hurt the site so badly in the first place, but I have a feeling I never will.