The Equal Employment Oppurtunity Commission has ruled that Title VII protects transgender workers from on-the-job discrimination. The EEOC pointed out that gender discrimination has been considered illegal under Title VII since the late 80's, and that employers cannot engage in sex-stereotuping. The status of transgender workers under Title VII had been unclear, with courts ruling differently on the issue.
See "EEOC rules job protections also apply to transgender people", Sam Quinones, Los Angeles Times, April 24, 2012