Source: USGS Landsat Mission

On August 21, 2012, all newly processed Landsat data will have updated filenames and metadata to better align all 40 years of Landsat data and to prepare for new data from the Landsat Data Continuity Mission that is scheduled to launch on February 11, 2012.

Details:
http://landsat.usgs.gov/Landsat_Metadata_Changes.php