i've messed something up

Hi, I'm sure it's something very simple that I'm missing. I must have buggered something up somewhere. It could be as simple as a single check box. I have a plane very close to the camera that acts as an overlay or hud. It uses a transparent diffuse material so you can see through the middle of it. Like a picture frame with no picture. When I turn the ambient light to full it looks fine. However I don't want ambient light and/or very little. The regular lights (point, spot, dir) don't reflect off of it? Even when I created a new light just to test and placed it right in front of the plane, it remains uneffected by the light. All other objects in the scene are responding to non-ambient light.

Any help is much appreciated.

Is there any particular reason you are using a plane in front of the camera as opposed to GUI or GUI Layout objects?