Women are definitely coveted amongst top MBA programs. I think most schools would like to see women make up at least a third of their class. Since there not as many women interested in MBA programs as men, I believe this leads to a higher acceptance rate for them than for the general population.
Below is a list of schools who list both percentage of women applicants and percentage of women in their most recent class in Businessweek.
Northwestern
Applications from women received: 29 %
Percentage of class who is female: 33 %
Duke
Applications from women received: 28 %
Percentage of class who is female: 37%
Michigan
Applications from women received: 25 %
Percentage of class who is female: 31%
Cal
Applications from women received: 31 %
Percentage of class who is female: 29%
MIT
Applications from women received: 23 %
Percentage of class who is female: 38%
Cornell
Applications from women received: 26 %
Percentage of class who is female: 30%
UNC
Applications from women received: 26 %
Percentage of class who is female: 25%
UCLA
Applications from women received: 32 %
Percentage of class who is female: 28%
NYU
Applications from women received: 35 %
Percentage of class who is female: 33%
Texas
Applications from women received: 26 %
Percentage of class who is female: 28%