Conditions on deriving Typeable

[George Russell]

Please satisfy my curiosity, what is it about

    data WrappedDisplayType = forall displayType .
       DisplayType displayType => WrappedDisplayType displayType
          deriving (Typeable)

that causes GHC 6.0 to utter

DisplayTypes.hs:135:
     Can't make a derived instance of `Typeable WrappedDisplayType'
     (`WrappedDisplayType' has existentially-quantified constructor(s))
     When deriving instances for type `WrappedDisplayType'

Obviously it doesn't like the existential quantifier, but why should
that be a problem for Typeable?