Why has not Pressflow been upgraded to 6.24?
I'm a developer at Four Kitchens.

Drupal 6.24 introduced changes to the bootstrap process, and those created a merge conflict with Pressflow. See David Strauss' comments here on Github.

The forks of Pressflow on Github may have the latest 6.24 but these changes have not been reviewed by Pressflow maintainers and no pull requests have been sent.

The forks also include patches from the Drupal issue queue to fix some serious issues that appeared in 6.24, and they are not the most recent patches in the actual issue nodes: 


drupal.org/node/1425868
drupal.org/node/1425260


So beware: Using a fork means you're using Pressflow upgraded to 6.x-dev essentially, except it's not even -dev as these issues are not yet adequately resolved; at least, not sufficiently enough to be accepted into Drupal 6.x-dev.

We're working on getting a full 6.24 stable version of Pressflow out. In the meantime, Pressflow 6.23 contains the security fix and was available when Drupal 6.23 was released. 

You can also follow Pressflow at https://twitter.com/#!/pressflow.