Andres Valloud attended a keynote at OOPSLA that covered type inference (using Haskell, sounds like):

Haskell's type classes are, as I understood them, a way to pass a sort of "method dictionary" so that type inference works. However, the type inference breaks when not all sends are "monomorphic". Well right, if all sends are monomorphic, then types are essentially method lookups --- hence my previous claim that good type inference engines are hard to come by

Go read the whole thing - Andres draws some conclusions, and they come from his experience in the field.

Update: With a tip of the hat to Giovanni, the link is fixed

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