Just as an anecdote, the fact that Haskell does this (infers type signatures for functions) is one of the things I don't like about it, because it seems to often infer things that are surprising to me. I think inferring types in most places, but enforcing explicit types at the level of method definition strikes a really good balance. I care about the types required to satisfy interfaces, but enjoy not needing to care so much in implementations.

IDE support for creating method signatures by inferring types would be neat though.