It's pretty obvious that just by being alive, we have an impact on the earth. "If I hadn't just eaten that animal, the last breeding pair of the species might have been saved" or "Damn, if I hadn't just eaten that plant, the last breeding pair of the species that lives exclusively on this plant might have been saved"
I can also see that nuclear waste seems to be providing one of the last refuges for "natural" life on Earth. There's this Russian woman, see, who's gone back to the Chernobyl region and taken photos of a place which has reverted back to nature, with slightly glowing animals. Check her site here. And I'm kidding, the animals don't glow or anything, and also, we won't know how long the animals manage to live and breed in the hot zones. But the hot zones do keep humans out, to a great extent.
So I can see the attraction of solving the problems of disposing of nuclear waste and maintaining natural forests in one solution. And yep, it would seem that this does neatly solve both problems, although I also say that probably just putting up radioactivity warning signs would have the same effect... %)
But I can also see radical, "fundamentalist' activists thinking along the lines of my first few paragraphs, and stealing material used to safeguard a forest, and spreading that among the ecosystem at large... In fact, I predict that before 2020, some misguided group of eco warriors somewhere is going to do precisely that, seed a large chunk of prime farmland or something like that with nuclear material or some other highly toxic material.
Ironically, the greatest effect on reducing human impact might be one of our more toxic byproducts, and the best of human intentions may result in the best of human extinctions...