Here’s a pretty decent list of potential ways to destroy the earth (physically demolishing it, not just screwing it up so that various things can’t live on it). How to destroy the Earth
The thing that’s not discussed in detail in the “Eaten by von Neumann machines” entry is the logistics of the ever-growing hoard. It’s not hard to imagine the first couple of generations of the machines, but I think some serious consideration would have to be made to start thinking about generations past about 20. The problem is, you have to gather the necessary mix of raw materials. Assuming that you’ve designed your machine to require the right mix in the first place, the mix isn’t uniform throughout the entire planet, and just transporting each machine to where it needs to be to gather the materials requires fuel, which has to be present in the right mix along the way to the destination… you might see what I’m getting at.
It could just be that I’m missing something obvious, but I think it would get pretty tricky to coordinate this aspect of that scenario, and further, your machines have to be intelligent enough to follow the plan, or to evolve enough intelligence in the meantime…
I think this gotcha applies to the ‘gray goo’ scenario of nanotech, too, so maybe gray goo isn’t so likely after all…