London's Dirty Bomb Tests
London is running a dirty-bomb drill. Mostly a movie-plot threat, but these sorts of drills are useful, regardless of the scenario.
I agree with this:
As ever, plain old explosives are the big worry. As for chemicals, compare the effects of the Tokyo subway gas attack (10 terrorists, five attacks each involving 1kg of hard-to-get sarin nerve gas, 12 dead total) with a typical backpack-bomb attack (London 7/7: four terrorists, four simple home made devices, 52 dead). Only a stupid attacker would bother with chemicals. Real pros like the IRA, for instance, never have.
Although with a dirty bomb, the media-inspired panic would certainly be a huge factor.