Many have been asking this question from us. The answer is:
Yes there will be the ability to post a message on a person's profile where only the Author and the message's new Owner(s) can see it. We will be implementing that for the upcoming releases.
I don't think we will be developing any social apps that mimics the behaviour of a desktop mail application. It is quite possible to do that in Anahita, however it is not something that we are interested in. That shouldn't stop you from doing it if that is what you really want.
We think in the age of real-time communication and the fact that many users are dropping traditional email to methods such as sms, tweets, etc. the messaging on social web will be going through a lot of changes and there is little merit in investing in the tradition methods of communication. Even facebook is now rethinking their messaging infrastructure: http://www.facebook.com/about/messages/