When I was in DC earlier this month, the security was very weird. You could walk within about 100 feet of the Capitol building without being stopped (see picture below). You can drive within a block. At the World Bank, you can actually drive a car under the front of one of the buildings (there are security guards but no concrete barriers). At the White House, you can not even walk within three blocks of the White House (see picture below). At various US department buildings (things under the control of the Bush Administration, security was very visible but on Sunday, there were no security guards around. (see picture below).
So why the difference? It appears the whole thing is not really about security. It is about creating paranoia among the general US population.