[picomancer]

I'm thinking the baseball spinning counterclockwise when viewed from above will curve to the pitcher's right (the same direction shown by Trefil).

Basically friction will cause the ball to push air molecules near its surface in its direction of spin. So air molecules in front will fly off to the left. By conservation of momentum, the ball will be pushed to the right.

The same effect at the back of the ball will push the ball to the left. But there would be fewer air molecules behind the ball, because that's the space which has just been vacated by the ball. Air molecules haven't yet had time to rush in to fill the space behind the ball at the same density as they fill the space in front of the ball.

This makes the rightward push at the front stronger than the leftward push at the back, causing the ball to move to the right.

Now I'm going to finish reading the article and see if my hypothesis is correct.

[picomancer]

Crud. I assumed the left and right side were symmetric and would cancel, but they're not -- the air's moving at different relative velocity on each side.