Federal Employees Paid Parental Leave Act of 2008 - Allows federal employees to substitute any available paid leave for any leave without pay available for either the: (1) birth of a child; or (2) placement of a child with the employee for either adoption or foster care. Makes available for any of the 12 weeks of leave an employee is entitled to for such purposes: (1) four administrative weeks of paid parental leave in connection with the birth or placement involved; and (2) any accumulated annual or sick leave.

Authorizes the Director of the Office of Personnel Management (OPM) to increase the amount of paid parental leave available to up to eight administrative workweeks, based on the consideration of: (1) the benefits provided to the federal government of offering paid parental leave, including enhanced recruitment and retention of employees; (2) the cost to the federal government of increasing the amount of paid parental leave that is available to employees; (3) trends in the private sector and in state and local governments with respect to offering paid parental leave; and (4) the federal government's role as a model employer.

Amends the Congressional Accountability Act of 1995 to allow the same substitution for covered congressional employees.

Amends the Family and Medical Leave Act of 1993 to allow the same substitution for Government Accountability Office (GAO) and Library of Congress employees.

Requires GAO to study and report to Congress on the feasibility and desirability of providing an insurance benefit to federal employees which affords partial or total wage replacement with respect to periods of qualified leave. Defines "qualified leave" as leave that: (1) is available by reason of the need to care for the spouse, child, or parent of the employee having a serious health condition or by reason of a serious health condition affecting the employee that renders such employee unable to perform the functions of his or her position; and (2) would otherwise be leave without pay.