I don't think that thinking through possible negative outcomes and taking action is indicative of pessimism at all. On the contrary, I think these things will boost your confidence in your ability to handle those situations adequately, and will make you more optimistic about the outcome.
To use tooq's example of a house fire, a pessimist viewpoint might go like this:
"Man, my house might burn down and then what would I do? I'd be screwed. I'd have no house, no money, my family might die and I'd be alone and grieving. I can't believe scientists haven't come up with fireproof houses and give them out for free!"
A "realistic optimist" on the other hand might say something like this:
"Well, I bought this house, but it could burn down. I guess I should get some fire insurance for it, then at least if that did happen, I'd get money back from them to buy another house. Of course, my family might get killed. I guess what I should do then is install smoke alarms and have an evacuation plan, that way they're less likely to get hurt. Now that I think about it, a fire isn't the end of the world and we'd probably get through it with those measures in place."
Now, a "delirious optimist" might simply say: "That'll never happen to me, so why worry about it?"
One could point out that neither optimist is worry about things he can't control (i.e. whether his house accidentally catches fire) but the realistic optimist IS thinking about things he CAN control (i.e. his level of preparedness).
