Thanks, guys. @bwnbwn: the search terms the traffic was coming in on before were the best I can describe them as random. The KWs were never stable enough for me to notice any one in particular. Besides, having only 20 visitors a day does not really entice you to spend any time analyzing that traffic, so perhaps I did miss one or two stable KWs. Anyhow, they are ALL gone now. The ones I do get *some* google traffic on look like mostly hackers googling my site for known exploits - not the actual themed KWs.
@netmeg: everything is possible I guess, but, man, what a potent algo, more dangerous than an outright penalty?! My other guess would be that the site might have gotten into a precarious position where all its pre-penalty links have been stripped, so it's like it's a brand new site without the benefit of the "honeymoon" period. Which brings me to another question: does anyone know what happens with the *good* remaining pre-penalty links: do they ever get used in the PR calculations again?