I’m the one who was working on this Summer of Code project. The patches have been sent to Duncan, but he hasn’t reviewed them yet. Note that my code works at the package granularity, so you won’t get any speedup when building a single package. I’m currently working on a parallel wrapper around ghc --make, which will solve this problem (I hope to get it merged into the mainline cabal-install eventually).
Update (Feb. 2012): Duncan has reviewed my patches, I need to incorporate his feedback and resubmit them. I hope to get this done before the end of this month.
Update (Apr. 2012): I’ve updated my patches in response to Duncan’s comments. The new code is a bit slower, but requires much less changes to the Cabal library.
Update (Jun. 2012): Duncan Coutts just merged the parallel branch into Cabal HEAD. Parallel install will be available in the next cabal-install release.
Update (Oct. 2012): cabal-install1.16.0 has just been released. This is the first official release that includes my parallel patches.