One of my favorite hard takeoff scenarios: the AI quickly figures out how to manufacture nanobots and does so at a scale to blanket the planet, thus becoming instantly omniscient with a global field of sensors, giving it a coherent view of everything in the world at every given instant in time from that point forward.
Also see: "metamorphosis of prime intellect" and "mind war: the singularity" and to a lesser extent "a young lady's illustrated primer"