I would be very happy to accept a bet with you on those odds if there's a way to sort it out. I'd define major as any attack with more than ten deaths.
I've added this prediction to PredictionBook: http://predictionbook.com/predictions/1565 based on the description at http://wiki.lesswrong.com/wiki/Bets_registry
I would like to propose this as a thread for people to write in their predictions for the next year and the next decade, when practical with probabilities attached. I'll probably make some in the comments.