diff --git a/checkout-nixos.sh b/checkout-nixos.sh
deleted file mode 100755
index 300deb196c6..00000000000
--- a/checkout-nixos.sh
+++ /dev/null
@@ -1,9 +0,0 @@
-#! /bin/sh
-nix-channel --add http://nix.cs.uu.nl/dist/nix/channels-v3/nixpkgs-unstable
-nix-channel --update
-nix-env -i subversion
-cd /etc/nixos
-svn co https://svn.cs.uu.nl:12443/repos/trace/nixos/trunk nixos
-svn co https://svn.cs.uu.nl:12443/repos/trace/nixpkgs/trunk nixpkgs
-ln -sf ../nixpkgs/pkgs nixos/pkgs
-ln -sf nixpkgs/pkgs/top-level/all-packages.nix install-source.nix
diff --git a/installer/nixos-checkout.nix b/installer/nixos-checkout.nix
new file mode 100644
index 00000000000..ce8038fffed
--- /dev/null
+++ b/installer/nixos-checkout.nix
@@ -0,0 +1,8 @@
+{substituteAll}:
+
+substituteAll {
+  name = "nixos-checkout";
+  src = ./nixos-checkout.sh;
+  dir = "bin";
+  isExecutable = true;
+}
diff --git a/installer/nixos-checkout.sh b/installer/nixos-checkout.sh
new file mode 100644
index 00000000000..8d1a13cafc2
--- /dev/null
+++ b/installer/nixos-checkout.sh
@@ -0,0 +1,28 @@
+#! @shell@ -e
+set -x
+
+# Obtain Subversion.
+nix-channel --add http://nix.cs.uu.nl/dist/nix/channels-v3/nixpkgs-unstable
+nix-channel --update
+nix-env -i subversion
+
+cd /etc/nixos
+
+# Move any old nixos or nixpkgs directories out of the way.
+backupTimestamp=$(date "+%Y%m%d%H%M%S")
+
+if test -e nixos -a ! -e nixos/.svn; then
+    mv nixos nixos-$backupTimestamp
+fi
+
+if test -e nixpkgs -a ! -e nixpkgs/.svn; then
+    mv nixpkgs nixpkgs-$backupTimestamp
+fi
+
+# Check out the NixOS and Nixpkgs sources.
+svn co https://svn.cs.uu.nl:12443/repos/trace/nixos/trunk nixos
+svn co https://svn.cs.uu.nl:12443/repos/trace/nixpkgs/trunk nixpkgs
+
+# A few symlink.
+ln -sfn ../nixpkgs/pkgs nixos/pkgs
+ln -sfn nixpkgs/pkgs/top-level/all-packages.nix install-source.nix
diff --git a/installer/nixos-rebuild.nix b/installer/nixos-rebuild.nix
index 52cc939eabf..2c8371a597c 100644
--- a/installer/nixos-rebuild.nix
+++ b/installer/nixos-rebuild.nix
@@ -1,9 +1,8 @@
-{substituteAll, nix}:
+{substituteAll}:
 
 substituteAll {
   name = "nixos-rebuild";
   src = ./nixos-rebuild.sh;
   dir = "bin";
   isExecutable = true;
-  inherit nix;
 }
diff --git a/system/system.nix b/system/system.nix
index 1b5069b014a..b3a888e6590 100644
--- a/system/system.nix
+++ b/system/system.nix
@@ -118,7 +118,10 @@ rec {
 
   nixosRebuild = import ../installer/nixos-rebuild.nix {
     inherit (pkgs) substituteAll;
-    inherit nix;
+  };
+
+  nixosCheckout = import ../installer/nixos-checkout.nix {
+    inherit (pkgs) substituteAll;
   };
 
 
@@ -192,6 +195,7 @@ rec {
     nix
     nixosInstall
     nixosRebuild
+    nixosCheckout
     setuidWrapper
   ];