For what it's worth, I got this while manually bootstrapping haskell-platform onto a GHC HEAD build and it turned out to be a complex interaction between unix, network, bytestring, and base versions.  Cabal's ability to report such things is limited, to say the least.  (More specifically:  I'd have to go through the bootstrap again to get the full details, but Cabal somehow managed to conclude that it needed a version of base with bytestring in it at the same time that it needed a modern version of base, and this was triggered somehow by newer versions of network which have subsumed the old network-bytestring package.  IIRC it actually made a certain twisted sense at the time.)