Send post receive json

[eluser]umbungo[/eluser]

Code:

$('span').click(function(){
    $.post("/", {'command': $(this).attr('id')}, function(data){
        alert(data.key)
    }, 'json');
});

The problem when i do this is that the 'post' is sent as json and not in the normal manner, but if i omit the 'json' note then the post section works but gives back a string which isn't interpreted as json. (i'm using json_encode on the PHP side)