A quick note on using Pressflow as the basis of a Drush build using Drush make can be a little confusing as the changes necessary to use the newer github-based repository aren't abundant. So without further ado here's what works:
projects[pressflow][type] = "core"
projects[pressflow][download][type] = "git"
projects[pressflow][download][url] = "https://github.com/pressflow/6.git"
projects[pressflow][download][tag] = "DRUPAL-6-24"
Note that the previous instructions around the web tend to be problematic because the tarball file contains a git repo which Drush make doesn't know to strop since it's getting it as a tarball instead of from git.