There are a number of examples on the jQuery.post API page that you linked to. They're about 1/3 of the way down the page. The last example on the page shows passing the 'json' content type in this method.

Here's a sample of doing a jQuery post -- and this works in jsFiddle without issue:

$(function() {

    $.post(/ajax_json_echo/, {html: stuff}, function(data) { alert(data.post_response.html);}, json);

});

I'm passing my data to the service using 'json' as my data type.

So, are you sure there isn't a server side error happening. Try running your call through Fiddler or Firebug and see what you get back. Also, what type of data is your service expecting?

Update your post with what you see and I'll update my answer accordingly.

Hope this helps.