I found myself in a situation recently where I wanted to have access to variables that would have been posted, in a the same structure as if the form had been posted and returned the JSON, using this jQuery plugin and Douglas Crockford's JSON library, I think I've done it!
The form below has elements in CakePHP FormHelper style names (
data[User][email], etc) so when they're posted to PHP you get deep arrays (
$_POST['data']['User']['email'], etc), which you can then turn into JSON (
You can download the script here.
You can test out the script using the form below: