NOTE: on lion (and snow leopard i suppose), make sure you are using a 64 bit install of ghc. Also, unless you are suggesting an edit to these directions, please go ask people on the relevant mailing list or wiki for help :)
NOTE: gtk2hs 0.12.2 won't build with ghc 7.4.1, but the current darcs repo for gtk2hs does build 
NOTE: on 7.4 there seems to be a problem when in ghci and drawing to a window, but not when rendering to pdf.  I don't know why this is, but if you do, let me know. I'll update this as i have more info

note that these directions probably can use some cleanup, but i'm choosing these steps so that rebuilding any haskell library doesn't require remembering ANYTHING :-)
(tested on ghc 7.2.2, assumes you have standard developer things installed on mac, like x11 and stuff)

1) cabal install gtk2hs-buildtools  #(this should work with any haskell platform install)

2) brew install cairo gtk gettext fontconfig

3) brew link cairo gettext fontconfig and all the other dependencies listed for brew's gtk formula
this is best done with
by hand running brew link for each of the items in `brew deps gtk` along with fontconfig and gettext.
some of these will already linked, and some won't be, so this command makes it simpler 
 # brew will complain, who cares, this makes your life easier (at least if you're living in a haskell world :p )

NOTE: you will need to make sure that all these Brew libs are built, and linked in

4) download libfreetype, heres a URL you can use http://sourceforge.net/projects/freetype/files/freetype2/2.4.8/freetype-2.4.8.tar.bz2/download?use_mirror=iweb
5) unpack libfreetype, and then run 
./configure ; make ; make install
this will install the static and dynamic library files for lib freetype in /usr/local/  (../include and ../lib) , which is what you'll want, though this will contribute to brew doctor complaining, but again, this is the easiest way

6) cabal install gtk

this should work sans complaints!

now you can eg 
cabal install chart-gtk  
and run 
this https://gist.github.com/1655252 example chart code either by building with ghc and running the executable
or by running main in ghc, and 
or
try out some other cool libraries like diagrams!