In a piece in Foreign Policy, Jamais Cascio goes straight at one of the things that scares me most about "geoengineering" — the potential, should such techniques be developed, that they will be used for less-than-benign ends.
Nuclear war scares the hell out of us, right? Why would it not scare us to think that any country on earth could, relatively cheaply, alter the entire planet’s atmosphere? Or even that a concerted group of individuals could? It’s nuts.
Anyway, read Jamais’ article: "Battlefield Earth." More notes on the piece here. The gist: geoengineering (or "terraforming") is inevitable, since no country will accept being the last to have the know-how.