Setting a date in the future I'm afraid won't mean it doesn't appear until that future time. It'll appear in the noticeboard straight away, but use that date in the footer of the item.

So, as you suggest, members would get the notification straight away, rather than at the future date.

You can set a future publish date for CMS items, and that would notify people at the time it goes live, but unfortunately not in the noticeboard/weblog app.

Thanks

Joe