Haskell bindings for Xft are on their way. They don't cover 100% of the API yet, because some of the API relies on the only briefly covered Xrender and FreeType APIs. But the whole thing is sufficiently usable to get nice FreeType rendered fonts in xmonad! Here is the pr0n.
For the moment my Xft patch for xmonad only covers Tabbed.hs, so you get Xft fonts in tab rendering. A few more pieces are missing in Prompt.hs, but I'm sure this will be covered soon.
darcs get http://clemens.endorphin.org/X11-xft/Enjoy.