Is there a module to track future revisions of published content in Drupal 6? I need to track a future revision of a published node that can be reviewed, edited, and eventually published over the currently published revision of the node. 

The idea is that you have a page that is published and public but you are able to start making edits on a copy of the published page that are not visible to the public. When you are ready you can then publish this revision over the published, public one.

This feature is provided by the (excellent looking) Workbench module. However, Workbench is only for Drupal 7. Is there a Drupal 6 module that provides this future revision support?
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.