Does this mean you are considering a type of tor/i2p integration into your client? As far as I understand, bitmessage relies on a few servers to store encrypted messages for a certain amount of time (2 days). Darknote would have this information permanently recorded on the blockchain..unless...
Regardless of the implementation, you're saying that your implementation of ring signatures can be utilized to create a short-life message? AFAICS that would involve pruning of the blockchain. Do you mean to say you're going to implement a pruning system into CryptoNote? Is this a reduction/reorginization of useful information stored in the blockchain, or the ability to cut off the blockchain before a chosen certain date?
Staying tuned now for sure

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.