>>That's after the fact though -- there are exploits that can end up on phones that silently forward text messages to somewhere else unbeknownst to the owner.
>
>Thus, if you are sure that your phone has none of these "exploits", you consider your phone messages secure?

Definately not, but as it is a separate channel it is better than using PC as well. You might look into encryping message services hosted outside 5eyes, Russia, China and so on.