...people continually reply to the email, thinking they are replying to the PM.
An item I sugguested to Rick was an email piping feature (or built in IMAP/POP3 mail client) that could process messages sent to it and "deal" with them appropriately (with an id in the subject line with how to deal with the message); but I don't think it was worthwhile for him at the time... Personally I think it would have been huge in the forum world as a feature...
Oh I'd really like the option too, if enough people feel similarly.
generally it'd be more of an admin option, to either send the full content of a message or a snippet of X amount of characters.