I have often wondered how long evidence of a technological society would last... it has nothing to do with environmentalism as Neanderthal Lee suggests, its just a thought experiment- fodder for sci-fi stories. For instance, If some species of dinosaur had developed as far as we have, would there be evidence?

What things that we have created would last 10's of millions of years?

Personally, I think there would be copious of our presence after 10's of millions of years- heck, we have found evidence of NATURAL nuclear reactors dating back 1.7 billion years ago (Look up the Oklo reactors) plus, if we were to disappear tomorrow, there would be the genetic modifications we have made to plants and animals that would be passed on... Cement, ceramics, and some metals might survive intact for the eons... it might take some digging, but I think our presence would be obvious even after a hundred million years. There is nothing wrong with such speculation, it isnt wishful thinking, its just pondering an interesting question.

Its sad that some people are too politicized to open their minds to such interesting questions...