I'm having a hard time understanding the intent behind "You shouldn't need to know Haskell". Do you mean in order to complete the tutorial and understand what Yesod does? That's probably true. Being someone who's been studying and using Haskell in personal projects for the past maybe 10 months or so, Haskell's syntax and fundamental design decisions (while spot on, in my book) are different enough from most programmers' skill sets that I don't think they'll make it very far writing a non-trivial Yesod app without knowing Haskell. I'm fairly proficient/enamored with Haskell now but I'm certain I would have crashed and burned if I got my start trying to learn Haskell from Yesod.

My first dose of Haskell was attempting to write an XMonad config. That turned out to be a frustrating, several hour long ordeal for me that had me write off Haskell as undecipherable, alien hieroglyphics for quite some time after that. I'd urge anyone reading the tutorial who likes what they see to put in the hard time to get a handle on the language, understand what its good for and what it isn't and then return to web programming with it.