So sometimes I want to retrieve data and dump it into FCKeditor textarea using AJAX. It's in an iFrame right now. Fortunately, I found this code on the web.

function showPostcardMessage_Response(obj) {
    var oEditor = FCKeditorAPI.GetInstance('message') ;
    oEditor.SetHTML(obj.content);
    // $('message').value =trimString(obj.content);
}

The function is SetHTML. You can also use InsertHTML to insert instead of setting it.