A simple change to admin/cabal-config.in that implements the changes discussed in issue 34.
This uncomments the libsubdir entry, thus adding a prefix of $pkgid/$compiler in order to avoid an interface file conflict when installing multiple packages that define the same module (reproduced with cabal-dev install citeproc-hs; citeproc-hs depends on packages using mtl and List, which both define Control.Monad.Trans.List).
cabal-dev install citeproc-hs
I should also note that I have only tried to reproduce the problem on OS X, as with those having problems in the thread referenced.
Based on the conversation in the issue, I'm not sure whether this is the right way to fix it, but I also don't have enough context to say what is. :) I'm in favor of merging this if there are no objections.
The problem was fixed by 54e7d93. Closing as redundant.