till the newest version 3.5, this could be still the problem of tinymce editor of WordPress. I just find it out today.
Most of time when we use visual editor to make a post or a page, in visual mode, the box will be blank and we can not see nothing and nothing we can edit. sometime we can see it but when we switch to text mode and switch back to visual mode, everything will disappears and the editor box is emty ( a blank box). I just see that in text mode there will be these codes automatically added:
<div><embed id="lingoes_plugin_object" width="0" height="0" type="application/lingoes-npruntime-capture-word-plugin" hidden="true" /></div>
When I delete these code and click "visual" button again , the visual mode works again. I work under Firefox+wordpress.
I do not know if it's a bug of wordpress.