I previously lived in a home in a new subdivision, and 8-years ago, they were building these homes with (plain 'ol) PVC laid for all ductwork running through the slab. Probably because it was more rigid than metal, and they could just pour the slab without worrying about collapse. Don't see why it would be a problem, after all it's made to stand-up to much more abuse when used as plumbing, than it would ever be exposed to as a HVAC duct. The only down side would be it's cost - 6" PVC is more expensive than 6" metal. Both are more expensive than simply using the enclosed 2x4 channel on interior walls.