For once, I am nearly aghast to hearing myself say, I think RSR is right (although not in his characterizations of you).
We can never know what would have happened if Bush had not implemented his policies (vague, I know, but you have to take them as a whole). It may be possible that many potential terror attacks have been thwarted before they even had a chance to develop. It is possible that terror attacks have been perpetrated elsewhere that would have been perpetrated here, but for our national security efforts. At the end of the day, not knowing what would have happened in an alternate universe, I can only say that Bush deserves a measure of credit for the fact that no major terrorist attack has been perpetrated in the US. That doesn't mean I agree with what he has done (I don't). It just means that I have to acknowledge that it might be working.
To use your analogy, one could say that Bush has done the equivalent of making sure that each home has a fire alarm or has outlawed smoking in bed. We can't say that a fire would necessarily have started anyway, but...
That said, I will blame Bush if there is an attack in the future, possibly even if it is after he leaves office. There can be no doubt that the war in Iraq has inflamed Muslim feelings against the US, and we have yet to see what will happen as a result of this.