I believe that everything is installed under /Library/Frameworks/GHC.framework and then symlinked. (At least, that's what I'm seeing on my system from the last version of the installer.)

See here for some details (may be out of date, in terms of SnowLeopard): http://www.haskell.org/haskellwiki/Mac_OS_X

As that link mentions, the platform also comes with an uninstaller at /Library/Frameworks/GHC.framework/Tools/Uninstaller.