According to USPS:

In June 2011, both the Postal Service and OPM agreed to seek a resolution  surrounding the Postal Service’s’ decision to withhold payments of its FERS contributions by submitting a request for a legal opinion from the Office of Legal Counsel (OLC) at the Department of Justice. Based on advice received from the Office of Legal Counsel at the Department of Justice, in Quarter I, 2012, the Postal Service is expected to resume the regular biweekly payments for its FERS employer’s contributions as well as remit all previously withheld payments, including the $911 million accrued at September 30, 2011.

The Postal Service had overfunded its FERS obligations by $6.9 billion at September 30, 2009, and sought to apply that overfunded balance to amounts currently due for employer contributions. OPM’s latest calculation shows that the surplus has grown to $10.9 billion as of September 30, 2010, the latest actual data available, and it is projected to grow to $11.4 billion by September 30,2011, assuming all employer contributions are made.  …however, it is OPM’s position that they are currently restricted by law from authorizing the return of those funds. As a result, various legislative initiatives have been introduced to resolve the matter.

Absent significant changes in the law, it is very likely that the Postal Service will default on the $5.5 billion prefunding payment to the PSRHBF due by November 18, 2011, and on the $5.6 billion prefunding payment due by September 30, 2012. Additionally, even if legislative changes defer or eliminate the $11.1 billion of PSRHBF prefunding payments currently due in 2012, the $15 billion debt ceiling will likely be reached in October 2012 when we are required to make a payment of approximately $1.3 billion to the DOL for workers’ compensation, because the Postal Service is expecting to resume payments to FERS, thereby exhausting the Postal Service’s external funding ability.