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Status
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).
Download
If you want to test, you can flash this 4GB SD image (DOS partition table + fat32 + ext4 rootfs):
magnet:?xt=urn:btih:0def3f6acb3bceddb22cb24098f58e40e2853ec2&dn=rpi2-nixos-4b09501f2-img.xz&tr=udp%3A%2F%2Fopen.demonii.com%3A1337&tr=udp%3A%2F%2Ftracker.openbittorrent.com%3A80
Then you should be able to nixos-rebuild any configuration.nix changes.
The image is the result of a "nixos-install" alone. No root password has been set, and it does not include a nixpkgs checkout or channel. In fact I (viric) created the FS into a NBD, not a real SD, to create this image.