How do I display a field that's set to "Hidden" by the content type in the template?
$node->content will be a string containing the generated HTML output of the node's fields that will be displayed.

In hook_nodeapi() the $node->content is already generated, but you can append the hidden field's value to it if needed, with something like this:

function mymodule_nodeapi(&amp;$node, $op, $a3 = NULL, $a4 = NULL) {
    switch($op) {
        case 'view':
            if($node-&gt;type == "mytype") {
                $node-&gt;content .= "&lt;div&gt;".$node-&gt;field_myfield[0]['value']."&lt;/div&gt;";
            }
        break;
    }
}