This blog comment describes what seems to me the obvious default scenario for an unFriendly AI takeoff. I'd be interested to see more discussion of it.
Let me point out that we (humanity) does actually have some experience with this scenario. Right now, mobile code that spreads across a network without effective controls on the bounds of its expansion by the author is worms. If we have experience, we should mine it for concrete predictions and countermeasures.
General techniques against worms might include: isolated networks, host diversity, rate-limiting, and traffic anomaly detection.
Are these low-cost/high-return existential reduction techniques?
ITT we talk about whatever.