Is the "registerSiteRoutes" hook screwing up my template path?
Craft will only check for templates in your plugin’s templates/ folder during CP and action requests. When you start routing to your controller action via a site-facing route, it’s no longer either one of those; it’s just a normal Site request. So you will need to point Craft to your plugin’s templates folder explicitly.

// Point Craft to this plugin's templates/ folder
$templatesPath = craft()-&gt;path-&gt;getPluginsPath().'myplugin/templates/';
craft()-&gt;path-&gt;setTemplatesPath($templatesPath);

// Render and output the template
$this-&gt;renderTemplate('_mytemplate', $variables);


Note that it will no longer be necessary to include your plugin’s handle in the template path.