The code in master as of June 2015 should be able to prepare a bootable NixOS for Raspberry PI 2.
There are still some drawbacks:
NixOS does not provide a /boot/config.txt (the FAT32 partition). Making NixOS work in the Raspberry PI 2 is mainly the result of the recent work of ambro718, Dezgeg and viric (#nixos@irc.freenode.net).