There's already a very primitive version of "type holes" available, namely, undefined. I realize it's not as advanced as what's to come, but I find myself using it somewhat frequently.
(For non- or fledgling Haskellers, "undefined" has any type, so if you define a function that plugs into your code and make its return value "undefined", then you can look at the type signature of the function and learn what the compiler proved about the type of that function. Pretty handy!)
Type holes themselves are already included in the HEAD of the GHC trunk, and will be included with the next release I believe. Undefined is useful, but you can't get the types of a specific subexpression easily -- with type holes, you can.
Slight upgrade: turn on the -XImplicitParams flag and then use ?nameGoesHere instead of undefined. Detailed type information will leak out in the errors or, if it can infer all of the types, the type of the top level expression that contains your ?implicit.
(For non- or fledgling Haskellers, "undefined" has any type, so if you define a function that plugs into your code and make its return value "undefined", then you can look at the type signature of the function and learn what the compiler proved about the type of that function. Pretty handy!)