This looks like a classic example of Poser's smoothing issues and render problems. some quick quesitons...

1. are you rendering with Firefly
2. if so, what are your settings
3. what are your smooth values on the parts of the plane; compare them with any normal smoothing commands used in your modelling app and you'll have the starting point for some tweaking
4. are you rendering ray traced lighting? if so, check the biases
5. are you rendering shadow mapped lighting? If so, increase the map size..

That's all I have off the top of my head.. flying blind here---Poser isn't open right now and I have to head out soon, but just thought I'd toss some things in as food for thought. -Les