Biowar
If these Terrorists are just straight up nihilists, maybe they'd enjoy the opportunity to kill as many people as possible. If these Terrorists are, oh, I don't know, Islamofascists bent on furthering the Universal Caliphate and the Jihadist International, I don't see how releasing a killer virus into the world will help them.
Give al-Qaeda its due: Osama bin-Laden is a civil engineer and Ayman al-Zawhahiri is a cardiologist. Certainly they've reflected on the fact that a killer virus would kill many, many muslims indiscriminately, regardless of whether they were Wahabbists or Cairo nightclubbers. Western civilizations, with a public health infrastructure and armies of epidemiologists and hordes of molecular biologists, are not likely to suffer nearly as much. Improvised cures, quarantines, and public sanitation measures would spring up in Tel Aviv a lot faster than they would in Mogadishu. This cannot have escaped the notice of anyone sophisticated enough to bioengineer a new mutant influenza virus.
And to give George Bush his due, he has sufficiently put the squeeze on al-Qaeda to where they really need to husband their resources. AQ can't go squandering money on a weapon which clearly is useless. Of course, I would in no way trust them to keep safe a test-tube full of killer virus; it might too easily fall into the hands of some unhygienic bearded fanatic with a less-subtle understanding of the geopolitics of jihad than that of old OBL. If he survived the earthquake. But because of the increasing attrition of Zarqawi's foot-soldiers and the slow-but-steady deterioration of his command and control, I can't see R&D of a weaponized killer flu virus as a budgetary priority for that bunch of assholes.