Hmmmm so for over a year I mucked around, trying to make cURL work, to use Mandrill on my GAE sites.
In the end, I have given up and simply started to use GAE’s built in Mail.
Was easy to setup, and to get HTML email templates to work I:
$message->setHtmlBody($header . $this->body . $footer);
had to use the setHtmlBody function. Easy as!