The Ocaml programming language has exactly the same problem, but the Debian Ocaml maintainers have automated the process and last time to Ocaml compiler version was updated over 95% of all Ocaml libraries were rebuilt within 24 hours.
Since I never built GHC from scratch, it is really hard for me to estimate how much effort would I have to put through to automate the whole building process. Anyway, automating the package building process is crucial for Linux distributions. One problem I see is the binary-only distribution of packages. This makes cabal-install incompatible with most distributions except, maybe, gentoo. The automation process would have to run through hackageDB tracking dependencies and compiling each needed library. Pretty hard stuff... Rafael Gustavo da Cunha Pereira Pinto