Here are a few women who disguised themselves as men and in order to fight. It is from one of the sites that George posted.

http://www.gendergap.com/military/USmil3.htm

Loretta Velasquez disguised herself as Lt. Harry Buford and fought at First Bull Run.

Cousins Mary (Tom Parker) and Molly (Bob Martin) Bell served for more than two years under the command of General Jubal Early and were both promoted. When their gender was discovered they were briefly imprisoned at Castle Thunder in Richmond before being sent home to their family.

Malinda Blalock joined company F of the 26th North Carolina regiment posing as her husband Keith’s younger brother Sam, they fought together for about a month, before he was discharged due to illness and she revealed her identity in order to accompany him home

Amy (some sources call her Anna) Clark enlisted in her husband Walter’s Louisiana cavalry troop disguised as Richard Anderson. They fought together at Shiloh and when he was killed she left the cavalry and enlisted in an infantry unit, the 11th Tennessee. She was wounded and taken prisoner in a battle near Richmond Virginia and sent home when her captors discovered she was a woman.