This stuff is a mess but it's improving, despite my cosmic psychic battle with Apple's demonic engineers.
install cabal2macpkg: http://github.com/gregorycollins/cabal2macpkg/tree/master
zap your GHC install: "sudo /Library/Frameworks/GHC.framework/Tools/Uninstaller"
"rm -Rf ~/.ghc ~/.cabal" (just to make sure)
install a fresh GHC binary package
make sure cabal-install is installed, run "cabal update"
run "sudo make" -- this will dump a bunch of .pkg files into a subdirectory of build/.
Using Apple's GUI packagemaker tool, package those up along with the "pkg/hp-postflight.pkg" file (make sure that one goes last!) into a haskell-platform-xxxx.w.y.z.pkg installer package.
build a .dmg file containing the GHC installer package and the platform installer package. If you can figure out how to bundle these together, PLEASE email me: email@example.com.
There's an Illustrator file containing the background I've been using in the img/ directory (mostly cosmetic)
Good luck (you'll probably need it)!