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I have to add this question. can scenes and saved figures done in poser 6 be rendered / imported into poser pro or poser 8?

From what I've seen, yeah. Poser Pro at least, although the new interface in Poser 8 will likely change your document size, being that the interface is all different.
I only did one test scene on that in Poser 8, and the document opened at a different size other than what it was saved out as in Poser Pro.
So if your old scenes use the document size as the render size, you'd have to manually change the document size in P8, or tell it to render at a specific size.

Of course, Poser Pro has gamma correction, and Poser 8 has tone mapping, so neither will render your old scenes the same. Those are both actually huge improvements though, although might take a little getting used to.
Far as the materials go, I don't think anything has changed enough to make a difference.
Poser 8 also apparently has normal mapping that actually works, as opposed to Poser Pro's normal mapping that is supposed to work, but doesn't.