Each version of Pressflow is API-compatible with the same major Drupal version. For example, Pressflow 6 is compatible with all Drupal 6 modules. Pressflow 6 also integrates the SimpleTest system from Drupal 7 and the CDN support patch.
Updated January 21, 2014
This branch is obsolete. Pressflow 6 now tracks the official Drupal git repository.
Updated May 25, 2011
Early work for Pressflow 7. Do not use for external projects because this uses Drupal.org's git repository, which lacks stable hashes.
Updated January 24, 2011