With clever use of revisions, we could eliminate the preview system
entirely from the codebase.

Here's how it would work. If a user clicks Preview, it creates a new
revision but does not mark it as the current revision. The preview seen
is the new revision rendered. If a user clicks Save, it creates a new
revision *and* marks it as the live one. Optionally, we could then
delete the interim preview revisions.

Another advantage to this is that a user can work on a node, preview it,
and have their work saved. If a user comes back and edits the node with
unsaved previews, they can choose to work from the latest preview or the
latest public revision.

I know Drupal 6 goes a long way to fixing previews, but this would
eliminate all inconsistency.