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.
This branch is obsolete. Pressflow 6 now tracks the official Drupal git repository.
Early work for Pressflow 7. Do not use for external projects because this uses Drupal.org's git repository, which lacks stable hashes.