http://www.apple.com/support/downloads/compressorupdate.html
If you are upgrading from Compressor 1.2, the Compressor 1.2.1 update includes the following features:
Improved application stability
Improved Preview window performance
If you are upgrading from a version of Compressor prior to 1.2, the Compressor 1.2.1 update also includes the following features:
Support for MPEG-1 output files
Support for industry-standard letterbox aspect ratios
Support for saving batches
Preview window improvements
Improved high definition to standard definition transcoding
Timecode included in QuickTime output files
Optimized export using Compressor from the Final Cut Pro HD timeline
If you are upgrading from Compressor 1.2, the Compressor 1.2.1 update includes the following features:
Improved application stability
Improved Preview window performance
If you are upgrading from a version of Compressor prior to 1.2, the Compressor 1.2.1 update also includes the following features:
Support for MPEG-1 output files
Support for industry-standard letterbox aspect ratios
Support for saving batches
Preview window improvements
Improved high definition to standard definition transcoding
Timecode included in QuickTime output files
Optimized export using Compressor from the Final Cut Pro HD timeline