That world already exists. The hard part is getting the isotopes necessary, but there are elements that will do the job. See my post: http://news.ycombinator.com/item?id=4067089
For a terrorist getting the elements would be hard, but a government would have no trouble, and given the long half life it's a pretty safe bet to assume this bomb actually exists somewhere.
Which was my point. Worrying about hypothetical android bomb carriers is silly when we live in a world where you can pack an atomic bomb in the space of a travelers briefcase. That allows for enough mundane methods of delivery to make suicide androids unnecessary. And as pointed out elsewhere, you don't have to have sentient AI to create something sophisticated enough to deliver a bomb.
EDIT: Struck the word "almost" from it's place next to silly in the above sentences.
For a terrorist getting the elements would be hard, but a government would have no trouble, and given the long half life it's a pretty safe bet to assume this bomb actually exists somewhere.