basically we do not need tor/i2p integration in client. Currently all messages are stored in blockchain, and recipient can always "receive" it with his private keys.
Yes we are thinking about short-life messaging implementation - and we do know the ways, there is no problem to do it. Short-life messages will not be included in blockchain.

After @Alias implementation you will be able to give a name to your address, like

@TheUsualStuf

So, to send you a message i only need to know your alias.
Aliases will be available via client, you will pay for an alias some XDN, miners will receive that rewards for aliasing.

The ways of message using:

Download client -> Mine some XDN with client (help network) -> Send message

#Mail #DarkTwitter #SocialNetworking .... etc can be implemented on XDN encrypted data channel.