It used to be that anytime there was an explosion on a bus (or other public place), the safe assumption was that it was a suicide bomber (or a mob hit). The past two explosions on buses (one yesterday, and one last year during Operation Pillar of Defense) both proved to have been explosive devices left on buses (and thankfully non-fatal).
Is this a new trend? Are terrorists getting less religious, and therefore less willing to lay down their lives for their causes? Is Israeli security getting better? Maybe it's all a coincidence?