Thrift Savings Plan Enhancement Act of 2008

Thrift Savings Plan Enhancement Act of 2008 - Revises requirements for participation in the Thrift Savings Plan (TSP) to require the Federal Retirement Thrift Investment Board to provide regulations for automatic enrollment of new participants to make contributions at the default percentage of basic pay.

Allows a default investment in an alternative fund or funds (in lieu of the Government Securities Investment Fund (G Fund)), as the Board may designate in regulations, if an election has not been made with respect to any TSP sums available for investment.

Requires the Board to include in the TSP, by regulation, a qualified Roth contribution progam (under which an employee may elect to make designated Roth contributions in lieu of elective deferrals under the retirement plan).

Authorizes the Board to permit the addition of self-directed investment options under the TSP if it determines that such an addition would be in the best interests of participants.

Limits such investment options to: (1) low-cost, passively-managed index funds that offer diversification benefits; and (2) other appropriate retirement investment vehicles for participants.

Deems a participant, in the case of a default investment in the G Fund or an alternative fund, to have: (1) elected to invest in such fund; and (2) executed the acknowledgement of risk otherwise required.