SARS will definitely have an impact on the economies of Asia and probably have some impact on the earnings of a number of US companies. That impact will be as much from fear of the disease as from the disease itself. Both will pale, however, when compared to the impact that will be falsely blamed on SARS rather than blaming the true culprit - a slowing global economy.
SARS will follow the Iraq war as the scapegoat of choice for lowered guidance and missed revenues and earnings, giving it a far more severe perceived impact than it will really have. Of course there is always the chance that it will spread more than is currently apparent, and if so it could cause great economic damage that would be widespread and real.