From NALC.org: Contract COLA =0%

There was no projected accumulation toward the fourth regular cost-of-living adjustment (COLA) for letter carriers under the 2006-2011 National Agreement following the January 15 release of the December Consumer Price Index (CPI). The next COLA will reflect the increase in the CPI, if any, between July 2008 and January 2009, and will be payable the second full pay period following the release of the January 2009 index.

There are eight COLAs included in the 2006-2011 contract.
From APWU.org: Cost of Living Adjustment (COLA) Update

For Employees Covered by the National Agreement and the Operating Services Agreement:

In December, the Consumer Price Index (CPI-W) fell to 610.075. After the fifth month of the six-month measuring period, and assuming the next COLA were based on the December index, the fifth COLA under the 2006 National Agreement and the Operating Services Agreement would be zero. However, the next COLA will be based on the January 2009 index point and will be effective March 14, 2009.

The fourth COLA ($1,477) was effective Aug. 30, 2008 (pay period 19-2008, pay date Sept. 19, 2008).