Your suggestion reminds me that installing soegtk caused an outpouring of messages about "ignoring libraries". So the problem may well have arisen at that point. I should probably look at the cabal packages for gtk2hs, or maybe contact the guys who put it together.
Ok, then what happened is that gtk2hs found the MacPorts Gtk+ libraries, which were built native/64-bit, and tried to use them in your 32-bit GHC environment. (gtk2hs does *not* come with Gtk+ libraries; you have to provide them from somewhere.)
So, either you need to find some prebuilt 32-bit Gtk+ libraries, or you need to rebuild the MacPorts libraries to be +universal so they have been 32- and 64-bit variants. That said, the latter will probably fail with missing symbols related to iconv (MacPorts' iconv library is not compatible with Apple's, which is used by the Haskell Platform build).