Hi,
I consider this a feature request though I'm sure its also reasonable to see it as a bug. :-)
The ability to have unpublished drafts is a feature I would really like myself and plan to implement very soon.
Currently, if you show the calendar and you navigate to a future date using the calendar, you can find and edit your future post. However users can also find and view them which is probably not desireable.
I will implement this so that posts with future start date are not visible to anyone except those with edit permission. I will add a link to view unpublished posts that will be visible and functional only for the users with edit permission and this will allow using the future date to establish a post as a draft. I will make the calendar navigation prevent seeing future posts by ordinary users.
Might even work on this today, its not a big job.
Thanks,
Joe