Ny Times Article on US Open 2009.

"The men and women play with different balls at the Open. The ball for men is Wilson’s 
extra-duty version, marked with a black Open logo, and the women play Wilson’s regular-duty 
balls, with a red logo.

The only difference is in the felt, according to Jason Collins, the global business director
for Wilson balls. The regular-duty balls have a tighter weave and fluff up less. Because of 
that, they play a bit faster than the extra-duty ball that the men use.

The extra-duty balls have longer fibers, Collins said, and fluff up a bit more during use. 
(Men often use regular-duty balls on clay, so that the dirt does not get into the looser 
weave, and during indoor tournaments, where static electricity can make the balls fluffier 
than usual, Collins said.)"