Source: USGS Landsat Mission Website
The Landsat project will begin processing MSS data on the Level-1 Product Generation System (LPGS) on September 15, 2010. This is the same system that creates TM and ETM+ products, and there will be increased geometric accuracy in many of the MSS scenes. The final day to order MSS scenes from NLAPS (the historical processing system) is September 1, 2010. There will be a 2-week time period where no MSS will be available for ordering or processing. MSS scenes will remain available for download until September 8, 2010, at which time all MSS data products processed with NLAPS will be deleted from the FTP server. After September 15, 2010, all MSS requests will be processed with LPGS.
MSS transition to LPGS timeline: 
September 1, 2010: Final day to order MSS products from NLAPS
September 8, 2010: Final day to download MSS products from NLAPS
September 15, 2010: First day to order MSS data products from LPGS (NLAPS processing will be no longer available).
More Information:
+ USGS Landsat Mission Website