I thought everyone knew this. Federer had declined slightly in 2007, but Mono ruined him in 2008. Guts and experience got him through it, but it was the worst Federer has played since 2002. The whole year was a disaster.
The 2008 Wimledon final though a great spectacle was nowhere near as high quality as the year before.
In 2009 he was actually pretty good at the slams he lost, but very poor at the French and poor in the Wimbledon final.
i am still a firm believer that his form already dropped big in 2007, because he started using the k90.. (and yes he won AO07 in a big way). From that moment on his forehand just didn't seem as flawless as before, more error prone. And yes, I know that many here say that his racket didn't change specs wise, but I think it did.
His mono wasn't severe though, also his form dropped off a bit in 2007 too. I think mono may have been part of it but I doubt it had such a massive effect on him when he still playing the AO though it.