(Even clean has a simple GUI. Is it that hard to provide a simple GUI like that to be installed by default?)
Why not provide two, that can be installed? Gtk2Hs and wxHaskell. You can bundle them by default, or download them, the difference is minimal.
In my humble opinion, in this context, GUI doesn't mean a library to implement a GUI application. It rather means an interpreter/compiler that provides graphical interface. Kaveh Shahbazian is a little bit wrong since there are some implementation with graphic interface like Hugs. But since Hugs is not a compiler but an interpreter, ones who are to develop a real world application will hardly choose it. Unless we are making in some specific fields e.g. a web application, we would often need to get a compiled executable one. In this point of view, there is no Haskell implementation with GUI environment for real world application development yet. This may not matter a lot since we've got some alternatives like Haskell in Eclipse, Haskell in Emacs, Visual Haskell, etc. But anyway I want some GUI built in GHC/GHCi too.