plastic is the answer. according to my friend and LEED consultant brad wood, the plastic that they use for this black piping is more flexibile, does less harm to the environment to make, and will last a long time with out leeching.
so, i am going with black plastic.
to learn more about mr. wood. check this out
he has been managing the Mainstream GreenHome, a LEED for a pilot project in raleigh, nc for the past two years for cherokee, the largest brownfield remediation company in the country. they just recently completed it and achieved platinum designation through LEED, the first in the Southeast! it is one of the greenest homes in the country. cool huh! so, i think he knows his stuff about the pipes is all i am sayin'.