This topic seems to have gotten dropped but not resolved. I have this same issue.
1) I create a new post and give it a future date of tomorrow, I check publish and save.
2) I click on edit for the post and the publish checkbox is no longer checked
3) I check it again and save it. When I come back it is still unchecked
4) If I go back in and only check the publish checkbox and save the xml file timestamp is not updated and neither is the flag.
5) If I go back and update the text and the checkbox the xml file is updated but still has IsPublished set to false.
6) No matter what I try the future dating of a post will not work.