WordPress MU

How can i output response as JSON in php? Are there any helper plugins/libraries?

Did you try searching php.net?

http://us3.php.net/manual/en/ref.json.php

I use the JSON functions quite a bit to interface with Google lattitude and also to my brothers website

Ensuring the correct headers:
http://snipt.net/cafespain/ensuring-correct-json-headers/

Outputting and encoding the JSON for a WordPress plugin
http://snipt.net/cafespain/wordpress-json-encode/

Thanks a million! Great forum by the way.