I have done some more testing, and this seems to be an editor quirk/bug.

For example, quitting and restarting Unity resolves this issue - until I change the scene view (e.g. zoom in/out), after which it stops working again. Making an unrelated code change in a script then fixed it again. And so on...

I am using Unity Free (2.6) on Windows Vista.

In the mean while, if you want to try another way while you're designing the game - try creating this C# script (CreateBGPlaneOnCamera.cs) and attaching this to your BG camera. Set nBackgroundLayerID to the BG layer number, and assign a texture to txBackground. It should get ya there quick and dirty.

When you're ready to stop playing around, just return to using the GUILayer approach, since I don't think that quirk/bug will happen in a built player.

using UnityEngine;
using System.Collections;

public class CreateBGPlaneOnCamera : MonoBehaviour 
{
    public Texture2D    txBackground;
    public int          nBackgroundLayerID;

    void Start () 
    {
        GameObject plane = GameObject.CreatePrimitive(PrimitiveType.Plane);
        plane.renderer.material.shader = Shader.Find("Transparent/VertexLit");
        plane.renderer.material.color = new Color(0.5f, 0.5f, 0.5f, 1f);
        plane.renderer.material.SetColor("_Emission", new Color(0.5f, 0.5f, 0.5f, 1f));
        plane.renderer.material.mainTexture = txBackground;
        plane.gameObject.layer = nBackgroundLayerID;
        plane.transform.parent = transform;
        plane.transform.localScale = new Vector3(Screen.width * 0.1f, 1f, Screen.height * 0.1f);
        plane.transform.localRotation = Quaternion.Euler(-90f, 0f, 0f);
        plane.transform.localPosition = new Vector3(0f, 0f, Screen.height * 0.5f / Mathf.Tan(camera.fov * 0.5f * Mathf.Deg2Rad));
        camera.farClipPlane = plane.transform.localPosition.z;
    }
}

you should put your new objects into its layer. like this:

gameObject.layer = 5; // 5 is the number of its layer