If Hong Kong cannot control SARS, and so far they cannot, then how likely is it that the rest of China will do any better?
No matter how hard the other nations of the world try, SARS will eventually find its way into the slums of the world's mega cities.
What happens when SARS is running through the slums of Mexico City? What happens to the United States when millions of Mexicans flee north across the border?
Millions of Mexicans with SARS will be in no condition to try to sneak into the US. But your main point is valid. We don't even know for sure when somebody becomes contagious.
SARS is already loose on the world, and since we haven't (and probably can't) stop international travel, it's going to continue to spread.
We can slow the spread of it. That's important, because with good medical treatment, the fatality rate is manageable. It's far too high, but it's not like smallpox or the Black Death of the Middle Ages.
As long as we keep the spread slow, hospitals can keep up with it. If millions come down with it all at the same time, all bets are off.