Designer viruses would not be attractive to normal terrorists, unless they thought that somehow they and their favored groups of people were immune.

Reminds me of a sci-fi story from a long time ago, whose title/author escape me, about a virus tailored for affinity to a particular race. It seems feasible, if the package includes RNA transcriptase that activates upon encountering particular markers for physical characteristics, such as melanin content in the skin.

There's a loose correlation between race and membership in Islam. A virus tailored to attack Caucasians would hit a minimal number of Muslims. A few Caucasian Muslims, like The Singer-Formerly-Known-As-Cat-Stevens, but the kind of terrorist who'd do this wouldn't worry about a little collateral damage. Especially to Cat Stevens...

On the other hand, the true nihilists who hate all humanity might find them just the thing. I could see a radical environmental group finally getting hold of,

I know that "nihilist environmentalist" is one of your ritual hyperbolic monsters-under-the-bed, but I just don't buy it. The vast majority of environmentalists recognize humans as part of the planet's ecology, not foreign to it; they want to reform humanity, not kill it. Sure, there are a few extremists who consider humanity to be the entire problem, but as Luddites, they're self-selected for lack of competency in fields like genetic engineering. It might make a moderately-good Jerry Pournelle butt-kicking story, but it's just not plausible in the real world. I'd focus on the more real threat of Islamist terrorists.

--Robert