"Most public entities have a policy of pre-funding their pension plans.... But, a commitment to pre-funding is not the same as a commitment to full funding.... And the distinctions are often obscured.... [T]he concerns that have led to today's 'full funding' regime in the private sector -- participant concerns about benefit security, creditor concerns about a possible default, and future investor concerns about compromised value -- also exist in the public sector. But, the mechanisms by which those concerns can be brought to bear and affect funding policy are much weaker in the public sector."