I think the issue with a unified inbox and the ability to push messages to different channels based on the recipient's choice is that a lot of time, the context of the message depends on the channel i choose to send it with.

The world of asynchronous and synchronous communication is going to clash in a very messy, destructive, apocalyptic Neo-versus-Agent Smith way.

For example:

1. SMS

When I send a message via SMS, it could be either to pass on information or initiate (continue) a conversation. Sometimes the content of the message isn't clear enough to highlight the context and when this happens between a boyfriend and girlfriend, the gates of hell can be opened, and the fury of a thousand suns going supernova is unleashed, usually on the hapless boyfriend (not a true story, I swear).

Or whether you should end the thread with 'ttys' or 'ttyl' or 'brb' as with IM protocol, and trust me, sometimes not continuing a SMS thread or not ending it nicely can lead to you having to spend money to say sorry.

Although I think SMS usage has been happening long enough that people generally tend to interpret context relatively well NOW, SMS communication sometimes do suffer from the problem of parties not knowing whether a conversation is supposed to be happening asynchronously or (relatively) synchronously.

If you have ever sent an SMS (usually to an attractive member of the opposite sex after a first meeting, or maybe a very prospective business contact) and waited for a few hours for a reply, frantically wondering if you had said something wrong, or that person wasn't interested in maintaining what looked like a blossoming relationship, then you understand how different perceptions of what mode a conversation is supposed to be in can sunder the social fabric.

2. Email

We tend to allow the intervals between subsequent emails in a conversation to be longer than SMS. After all, there is the general understanding that accessing and replying to one's email is harder than receiving and sending messages via SMS because of the ubiquity of the mobile phone versus Internet access on the move and the ease of entering a long form email versus sending a SMS. Also, it is easier for most to type long message when emailing than when sending a SMS due to the nature of the clients and where we actually do it (i.e. on the desktop). I usually switch to a more focused mode when reading emails because they generally are about work and are usually longer.

Subject headers in email are a great way to delineate threads. An email with a new subject is the start of a new thread.

3. Instant messaging

Instant messaging is clearly generally expected to be synchronous. When instant messaging, there are socially acceptable standards to start and end a conversation. The signaling has been learned.

Is it going to be impossible to learn the new social rules of engagement? I don't believe it will be so. However, I do believe that the lack of clear signaling of the context of a conversation (i.e. synchronous versus asynchronous, start of a new topic ...) could be disastrous.

Note: Maybe only some of us have to relearn. It could very well be the younger generation already know how to contextualize the messages without use of the various channels, subject headers ... If so, I'm curious how they do it.