I had to read through the Jekyll source to figure this out since I didn't see this feature documented anywhere (I'm sure it is though) so I'm going to document how to filter the display of posts in Jekyll by a published date.
For example, you write a post now but don't want it to display until a certain time tomorrow.
In _config.yml add/modify the following:
future: false # This will tell jekyll to not display future posts
In the post header add a date attribute with the date and time you want the post to publish:
date: 2012-03-28 18:31:00