The following is what I perceive to be a regression from ghc-7.0.4 to ghc-7.2.1 and ghc-7.2.2.

On NixOS, all packages are installed into separate directories in the Nix store. With ghc-7.0.4, the following works:

$ ghc -package zlib -e "print ()"
()
$ ghc-pkg describe zlib | grep library-dirs -A 1
library-dirs: /nix/store/vnflri0w0fadiqwv1j6w6gxw00paav6a-haskell-zlib-ghc7.0.4-0.5.3.1-profiling/lib/zlib-0.5.3.1/ghc-7.0.4
              /nix/store/0jmzlnayh2p06vlgmiiyaj3xppc6l9lr-zlib-1.2.5/lib
$ strace ghc -package zlib -e "print ()" 2>&1 | grep libz.so
stat64("/nix/store/vnflri0w0fadiqwv1j6w6gxw00paav6a-haskell-zlib-ghc7.0.4-0.5.3.1-profiling/lib/zlib-0.5.3.1/ghc-7.0.4/libz.so", 0xb5e94490) = -1 ENOENT (No such file or directory)
stat64("/nix/store/0jmzlnayh2p06vlgmiiyaj3xppc6l9lr-zlib-1.2.5/lib/libz.so", {st_mode=S_IFREG|0555, st_size=91092, ...}) = 0
open("/nix/store/0jmzlnayh2p06vlgmiiyaj3xppc6l9lr-zlib-1.2.5/lib/libz.so", O_RDONLY) = 7

However, if I try exactly the same with ghc-7.2.1, I get:

$ ghc -package zlib -e "print ()"
<command line>: can't load .so/.DLL for: libz.so (libz.so: cannot open shared object file: No such file or directory)
$ ghc-pkg describe zlib | grep library-dirs -A 1
library-dirs: /nix/store/6zjd86g35dpxl5r6zrr67nnmf00qalkr-haskell-zlib-ghc7.2.1-0.5.3.1-profiling/lib/zlib-0.5.3.1/ghc-7.2.1
              /nix/store/0jmzlnayh2p06vlgmiiyaj3xppc6l9lr-zlib-1.2.5/lib
$ strace ghc -package zlib -e "print ()" 2>&1 | grep libz.so
open("/nix/store/ri2nlzb1lsi0d1y7nb8dkhzhsnyd08k4-gmp-4.3.2/lib/libz.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/nix/store/nmliqd429c0s4sdlbk0394zdfx86gvf9-ncurses-5.7/lib/libz.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/nix/store/m9p1r0p6qlaw5wy5hnwpii323la3s8j3-glibc-2.12.2/lib/libz.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/nix/store/m9p1r0p6qlaw5wy5hnwpii323la3s8j3-glibc-2.12.2/lib/libz.so", O_RDONLY) = -1 ENOENT (No such file or directory)

The directories listed by strace seem to be the ones used for building GHC itself. The directories in the package configuration file seem to be ignored.