actually i got to the cause of the issue.
if you feed the textarea text attribute with an tag that has a valid src url, then for some reason flex will try to render everything as html.
Eg, try this:
<mx:TextArea id="textArea" width="100%" height="90%" text="<img src='http://url-to-a-valid-img"/>
and instead of it rendering it as raw text it will render it as an html.
any idea?