Washington is still preparing for what will probably be a very long campaign. This delay is due to several reasons:
First, the Bush administration is still searching world wide for reliable coalition support for its war on terrorism.
Second, the first strike is important because it will shape US relations with the rest of the world, the most important of which is the Middle Eastern nations. It mustn't be hasty so as not to undermine the fragile confidence of allies or fuel wider hatred among Islamic and Arab nations.
Third, the fact that Afghanistan is a land locked country hampers preparation. In terms of logistics, the US must secure land bases for a sustained coalition air strike or provide in-air refueling for carrier based planes. Both of these choices are limited, for reasons I won't get into here.
Fourth (this is my opinion here), I believe that Bush wants to avoid a Clinton style strategy of launching ineffective cruise missiles against "some empty tents" and thus also avoid undermining shacky relations with the Arab and Muslim world.
Remember, it took Washington 6 months to gear up for Operation Desert Storm, and the ramp up for this war may take just as long.