I haven't really thought about GUIs much in the last decade, but if one were to embark on such an endeavor, I think an appropriate starting point would be SDL with OpenGL.

There would be no very little API structure to hinder the pure functional approach, and it would be portable to a wide variety of platforms. SDL includes support Linux/Win/OSX and now the iPhone. Also you'd have unofficial support for another few platforms.

The 1st step would be to build a vector graphics lib for Haskell on OpenGL, but after that, perhaps much of Fruit could be fitted in. (Just a WAG, never heard of the API before now)

Well, I've started something like that: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/graphics-drawingcombinators. There are some issues with that library: the semantics are not quite right, and its internals are complicated and not easily modified. I want to try again at some point.

However, I think before the 1st step, the SDL bindings ought to be easily installable and usable on Windows -- not first assuming Just The Right Setup.

Luke