Sending a message means that there is a receiver for this message. senddata RPC doesn't add a receiver into tx but as soon as any tx has a data field you can do this (=> needs a bit of coding but doesn't need a hardfork).

I don't think that messaging (like IM or e-mail) is a very good idea for the blockchain: IM/e-mail is a one-to-one communication and data in blockchain is more suitable for one-to-many communication model.