Just a small point: in page.tpl.php, there's a section in the descriptive comments entitled "Available variables", followed by a long list of them.
$node is not in the list, but it is available to page.tpl.php.
How about a line something like
* - $node: The node object, if the current page is a single node, including all properties such as e.g. $node->type, $node->created. If the page is _not_ a single node, $node is empty.
maybe under "General utility variables"? as it's not called directly in Zen's page.tpl.php.
I've put "minor" inasmuch as the lack of this doesn't actually stop anything working, but I do think it's a bug, as it misled me for a while into thinking $node wasn't available there.