Seriously, I think a lot of people in this discussion are overlooking the fact that waste at its core could be a resource.

We're far from this being a problem, but eventually in a multi planet society we might have to start considering the impact of planetary erosion. Everything from industrial waste to human waste being transported off planet is elements of the periodic table being removed from what was a closed system.

Again, scale comes into play: it would take a long time and tremendous amount getting off world, but I think writing off any potential exploitation of resources to the point of exhaustion should be taken seriously. Just ask the world's fresh water supply...

Edit: This is also why I think mining asteroids should be higher on our list than settling Mars. Not to mention spinning up a NEO industry would make planetary travel a lot easier.