http://www.farmtek.com/farm/supplies...oductId=416869
It's 6-mil. The UV inhibitor that they put into it is stronger than the process they use to make the seams.
And yes, pvc pipe will eat poly, so will treated lumber. White latex paint as a barrier is the answer.
edit: I just noticed that link says:
UV stabilized for up to four years of film life.
They must have been talking about the seams and not the plastic.