I realise this is an old topic, but it doesn't seem to have been fully answered.
Firstly, this is NOT a problem with the ART renderer.
As stated previously, the highlighted areas in the posted images are showing at their full ambient value (grey in most cases). This is because these areas are not receiving any diffuse light AT ALL, which leads to the conclusion that shadows from the included light sources have been set to 100%. Faces of objects not receiving ANY diffuse light can only display at their ambient value, so if a chair or a bed is causing total occlusion of an object (ie casting a 100% shadow), all faces in that shadow will only display their ambient colour, regardless of which 'way' they are facing in respect to any lights present.
Ambient light is represented as an artificial glow taken by objects in 3d applications. This is a crude, but usually effective, representation of what ambient light actually is, which is real light bouncing off a million different surfaces to the extent that it no longer has a 'direction'. You can see this effect yourself by rendering a scene with only one black light in it - you will still 'see' all of the objects in your scene, but they will be completely 'flat', and completely unaffected by where the black light is located. Diffuse (scene) lighting is what gives them 'volume'. If you want to use 100% shadows, you really need to be setting all of your ambient colours to 0, and placing your own ambient light sources in your scenes.
The easiest way to 'fix' this would be to reduce your shadow level from all lights, generally a value of about 95% or just above looks best in this sort of scene.
Your alternative (as stated) is to have an additional, low level light source in the centre of the room. Not adding any shadows to this light will provide the 'volume' you are looking for, even on the surfaces which are not apparently facing the light (eg under the bed, or the side of the night-stand).
Also as stated, excellent work on the models Kyle, your renders look superb, I hope you get them looking exactly how you want!