How many times does it have to be said that Iraq War II turned that country into a proving ground for terrorists? They have been able to fine-tune their cowardly and murderous tactics in preparation for something like this.
It appears that this was a highly coordinated attack with four, possibly as many as seven different explosions taking place in a close timeframe, though details are still being hashed out, of course.
Only a fool would think Al Qaeda (or whatever splinter group is responsible for this) would not feel emboldened now to try something else in this country again.
(by the way, great stuff at Atrios and especially The Daily Kos today about this tragedy.)