Is there a module to track future revisions of published content in Drupal 6?
In D6, you can do this with Revisioning and Rules.  You may also need to throw views, etc. into the mix.

Revisioning provides the ability to add pending revisions to content types.  You can set it so that you are tracking only one pending revision per node or you can even track revisions of pending revisions.  Revisioning provides actions and events to rules so you can make your own workflow.  It's not as easy to set up as the Workbench module, but I've implemented a similar workflow with these two modules.

There's also the Workflow module but I don't have any experience using it.