Yep, I have the lastest Poser SIX patches installed... I'm spelling it out this time to avoid typos. ;-)
 
The material settings for the ground are as close to the background as possible: the diffuse color, specular color, ambient color, etc are all the same. The only difference is that the ground material has a lot more options than the background material (displacement, bump, etc)... though these are all set to 0 (for values) or black (for colors).

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