From TinyMCE, jQuery and Ajax forms:
TinyMCE Form Submission
When a textarea is replaced by TinyMCE, it's actually hidden and TinyMCE editor (an iframe) is displayed instead.
However, it's this textarea's contents which is sent when the form is submitted. Consequently its contents has to be updated before the form submission.
For a standard form submission , it's handled by TinyMCE . For an Ajax form submission, you have to do it manually, by calling (before the form is submitted):
tinyMCE.triggerSave();
$('form').bind('form-pre-serialize', function(e) {
tinyMCE.triggerSave();
});