Attack On America
The terrorist aren't quite sure what trigger President Bush might pull if the USA has a major terrorist attack. This Bush factor forces caution on their part as to when they next attack the United States. With my meager power to surmise the Islamic mind (although I taught, trained and supervised Arabs in the Mid-East) they will want to hurt Bush as much as possible.
The Iranians (Shiite) poked President Carter in the eye when they released the embassy hostages the day he left office. The Sunni influenced the elections in Spain by bombing Madrid trains four days before the Spanish elections. Looking at just that when the attack comes may depend on the political polls just before November 2008. If not then I would suggest (guess) near the time of the swearing in of the next U.S. President.