Suggest that a copy of the PM, including the message text, is emailed when the PM is read.
And that a similar email is generated when sending a PM.
This will make it possible to have a complete local backup of PM's, in an easier way than the current archiving function.

Not something I'd like to see.

With the current pm system the text doesn't ever leave your server to go wondering around smtp's ....thus it is more secure.