I'm not sure if this is desired behavior or an issue. After successfully getting NixOS installed and running on my laptop, I decided to copy my /etc/nixos/configuration.nix to my desktop. To my surprise the following happened:

1. At the login screen, instead of "Session: none + xmonad" at the bottom it just stated: "Session: xterm"
2. After logging in, it seemed like my display manager was just a single xterm (hard to describe, but I had a terminal in the top-left of my screen, and then default / generic background for the rest).
3. running xmonad --recompile gave me a ghc-related error
4. running ls -ln \which xmonad`` gave me what looked like the desired result (xmonad was properly installed).

Steps to Recreate Issue

1. install NixOS
2. use this configuration: /etc/nixos/configuration.nix
3. login as aaronlevin

Steps to "Fix" / "Hack" Issue

To fix this, I had to add the following line to my /etc/nixos/configuration.nix:

  services.xserver.desktopManager.xterm.enable = false;

Desired Behaviour

If I set xmonad as my window manager, I shouldn't have to disable xterm as a desktop manager. Further, setting desktopManager.default = "none"; should work disable xterm.

Other Notes

Note that this line was absent from my laptop configuration. I believe some kind of referential transparency was broken (perhaps on purpose), because when I first setup my laptop I included that line having seen it on a blog somewhere. I later removed it.