Joomla is a great CMS – one bad thing about it – the documentation just sucks.
I have recently been using Zend AMF with Joomla in a custom component to communicate with a Flex Application.
To get the output back with the right content-type headers you need to
add the following before you return the amf content.
$document =& JFactory::getDocument();
If you dont do that it might still work, but you wont be able to see any debugging information in Service Capture for instance.
Flex itself seems to be clever enough to decode even a text content type properly.