It's simple enough, but worth a summary:
- When an item is past its expiration date, it's marked "expired" in red in its document byline when viewed.
- An item whose publication date is before the current date doesn't get extra text in its byline.
- In both cases, the item is "unpublished", which is not to be confused with a workflow state.
- It merely means the item doesn't show up in listings and searches.
- These listings include folder listings.
- However, the owner of the item will keep seeing it, which is handy because you like to know what you have lying around in your site.
- The permission that controls this is Access inactive portal content.
- Expired items in a folder are marked as such when viewing the folder_contents.
- There's no quick way of seeing if items in a folder listing are not yet published.
- When you set an unpublished item as the default view for a folder, that item will be shown.
- Unpublishing an item doesn't have any effect for admins. They will always see unpublished items in their listings and searches.
- Giving another regular users rights ("can add", can edit", "can review") on the item doesn't make it any less unpublished for those users.
- A practical way for a non-admin user to access an unpublished item is directly through its URL.
(2012-10-07: added this summary to http://plone.org/documentation/manual/plone-4-user-manual/adding-content/setting-basic-properties)