Show node if exists, else show node creation form
I ended up getting this to work with views, date and some custom code.  Basically I created a page view that with a conditional filter for a post date in YYYYMMDD format (a default argument provided by date), if one didn't exist I added some code in the "No Results" field that would load the node creation form with basically:

$form = node_add(arg(0));
print drupal_render($form);


So a url of /story/20110915 would show the story from that date if it existed, otherwise it would show the node/add form for the story content type.

In the final product I actually had the view load an .inc file from my custom module in the "No Results" field (also much faster than editing the view every time while developing). But I think this gets the basic point across. 

Sorry @ram4nd if I made it seem like I didn't want to write code, I just was looking for the best approach.  So far this seems like it is pretty solid.