Having pottered with bagginsbill's problem scene (I moved the infinite light flat on to scene and gave it -45% Xrot, Y and Z 0) I found some more unusual behaviour and a work arround.

As already stated there is a scene bounding box issue with Infinite Lights using Shadow Maps where at a certain point the light just casts a huge shadow into the scene. I found that this bounding box was approximatly 0.1 PU in size (with the light flat on to the scene). However if I tried to find the exact point the broken shadow would actually creep into the scene. (see attached).

I found that adding in a 2nd item just outside the critical distance (0.11PU) and removing render attributs wouldnt fix the problem. HOWEVER if I kept ALL its shadowing attributes but moved it below the ground plane out of sight the scene then rendered correctly. Not a fix but a simple work arround.