One of the things one can't help thinking about when one looks at the horror in London is just how much worse it could have been, a point Andrew Sullivan quotes the Economist on.
Unfortunately, given that systems disruption appears to be Al Qaeda's primary tactical approach to battling Western powers, the eventual use of nastier methods seem all but inevitable. Although the Bushies like to raise the spectre of a mushroom cloud (a symptom of the pervasive, misguided Cold War era mentality that dominates their thinking), the much more likely possibility is the use of some sort of "dirty bomb."
According to this useful primer on dirty bomb effects, a simple way to construct such a bomb would be to take ten pounds of TNT and combine it with either a pea-sized pit of Cesium-137 (found in some medical guages) or a 1x12 inch rod of Cobalt-60 of the sort used in food irradiation facilities. Although these materials are hard to handle, they are far more readily available that actual nuclear weapons, and therefore seem overwhelmingly more likely to tools that "open source terrorists" will seek out.
The above site presents disturbing graphics of what the geographical scope of destruction for such attacks would be. Although a cesium-laced attack would destroy many city blocks, a cobalt-laced bomb would be destructive at a scale that is hard to conceive: "It would be decades before the city was inhabitable again, and demolition might be necessary. If such an event were to take place in a city like New York, it would result in losses of potentially trillions of dollars."
In fact, the Federation of American Scientists provides a graphic of the impact of setting off a Cobalt bomb in lower Manhattan on a day with a south wind, by comparing it with the destruction around Chernobyl. Everything below Midtown would be irradiated at the same level as the permanently closed zone around Chernobyl, and all of Manhattan would experience the same radiation level as the permanently controlled zone around Chernobyl. Everyone as far north as Westchester would experience a half percent increase in cancer rates.
As I say, as grim as it is, it seems all but inevitable.
Too bad Bush decided that the right way to deal with this threat was to chase demons in the Mesopotamian desert instead of securing all our chemical factories, container ports, hospitals, and radiological laboratories.