Thanks David. I don't understand the relation between the library version numbers and what you see in /usr/lib. For Slackware/Salix 14.0, I think it's libffi.so.6. When I built the library from that link, it produced '.1':
vanilla[libffi-1.20]$ find ./ -name libffi.so.*
Regarding the ghc version, that one [EDIT: 7.4.2, as at slackbuilds.org] works with the latest version of Haskell platform (in fact the *previous* version of ghc is officially recommended, but that one works). And it works with xmonad.
However, maybe a more recent ghc would avoid problems with ancient libraries. I think I'll leave that for the next release cycle, though.
I'll keep searching (the very fact that you found this file inspires me to persist), but if anyone happens to have specific info, I'd be delighted.