tedster - 8:56 pm on Mar 25, 2011 (gmt 0)
If any site manages to get to PR8 they were doing a lot of things right. I notice that you said the loss came this week - so that doesn't sound like the Panda Update to me. I'd lean first toward either cloaked, parasite content being hosted or a very nasty competitive attack. Maybe someone on staff has been selling links on the side, too - hey, it happened to Google once.
I'd also drill down into which keywords and landing pages took the biggest hit. You need to find a loose thread and just start unraveling it. And yes, the Site Review forum in our Supporters Area [webmasterworld.com] may be the best place to get real, actionable help.