With respect to federal claims brought under Title VII, the EEOC has issued regulations on the subject.
Link: http://edocket.access.gpo.gov/cfr_2003/julqtr/29cfr1604.11.htm
29 C.F.R. 1604.11(e) provides:
An employer may also be responsible for the acts of non-
employees, with respect to sexual harassment of employees in the
workplace, where the employer (or its agents or supervisory employees)
knows or should have known of the conduct and fails to take immediate
and appropriate corrective action. In reviewing these cases the
Commission will consider the extent of the employer’s control and any
other legal responsibility which the employer may have with respect to
the conduct of such non-employees.