ghc-5.03でたよー！
変更点をコピってきました。

・ The type system now supports arbitrary rank polymorphism,
　　given appropriate type annotations.

・ Heap profiling has had a major overhaul and now supports
　　retainer profiling and biographical profiling ala nhc98.

・ Major improvements to the native code generators.
　　You can now compile any and all code through them,
　　including the Prelude.

・ The FFI syntax has been updated to match the latest version
　　of the FFI Haskell 98 Addendum.

・ newtypes support deriving *any* class for which the underlying
　　type is also an instance.

・ Linear implicit parameters: a highly experimental feature.