@dtweney,
Having had a brief look at the code for the Publish to Schedule it seems that it allows the post to be published first (thus allowing Subscribe2 to send) and then changes the posts date and status back to future again, this will probably mean Subscribe2 will actually send 2 notifications, one at the time when you hit “Publish to Schedule” and one again when the post actually becomes visible on your site.
I cannot think immediately of a way to make these 2 plugins work together that I could use in the core code without breaking things for others. If you really want to use them both together you could try commenting out some of the Subscribe2 code.
Look in the classes/class-s2-core.php file for these lines:
add_action('new_to_publish', array(&$this, 'publish'));
add_action('draft_to_publish', array(&$this, 'publish'));
add_action('auto-draft_to_publish', array(&$this, 'publish'));
add_action('pending_to_publish', array(&$this, 'publish'));
add_action('private_to_publish', array(&$this, 'publish'));
add_action('future_to_publish', array(&$this, 'publish'));
Comment out all of those lines with the exception of the last one:
//add_action('new_to_publish', array(&$this, 'publish'));
//add_action('draft_to_publish', array(&$this, 'publish'));
//add_action('auto-draft_to_publish', array(&$this, 'publish'));
//add_action('pending_to_publish', array(&$this, 'publish'));
//add_action('private_to_publish', array(&$this, 'publish'));
add_action('future_to_publish', array(&$this, 'publish'));
This will make sure that Subscribe2 only sends posts for future posts.