It's important to understand what identity isn't: Identity is not a password, it's not root access, it's not your calendar, it's not your email, it's not a technical achievement, it's not your location, it's not a user account in a system, it's not your contacts and it's not a feature.
So, what is identity? I think in its most basic form, your identity is the product of how you manage your attention and others' access to that attention. Those areas where your attention is focused assemble to form a set of experiences that shape and influence where you'll direct future attention. But that attention is interrupted all the time by people, events, things, desires, boredom, weather, etc. and that process of interruption is, largely, contained to physical space because that is a natural gate on access.
Then there's the phone. The "phone" part of the mobile phone is important not because of the voice communication it enables, but rather from the habit and etiquette that the ringing bell created in society and the direct access it grants to the caller. It's the promise of instant communication at the cost of having attention interrupted and redirected. The key to unlocking that attention is a semi-random sequence of digits which you can give to someone else to indicate that the person now has permission to interrupt you and to access your attention directly.
Email works so well because it is another opportunity for access and people have formed a collective habit of actively directing attention toward their inboxes at regular intervals. We have all agreed to walk to our computers and check the new mail indicator and are generally addressable through a combination of a username and domain. It's not as insistent as the phone, though, and provides just enough lag to enable some measure of control over granting access. Twitter and Facebook have feeds which abstract away both the To: and Subject: fields of email and represent two very different networks but are nonetheless an evolution of the habits email created. Facebook further improves the method of connection through friendship and the use of real names with the network itself providing necessary disambiguation.
Currently all these disparate forms of communication are competing and in terms of mobile, specifically, have been fashioned into apps that exist without hierarchy on top of an identity agnostic framework. Identity is largely misappropriated or at most a small component of some percentage of those apps. Then there are the notification systems which allow us to manage certain types of incoming information, but the experience is still essentially a channel for the applications' own interests: ignoring, instead of reinforcing, the user's identity.
It cannot be ignored, however, that there are benefits to that competition. With several different communication channels, it's easier to prioritize who gets direct access (texting, calling), who gets secondary access (email) and who gets excess access (feeds). Another issue is how quickly these systems become overwhelming and how easy it is for well intentioned users to dig themselves into an experience hole. But these are tractable problems when the design principles are focused around identity and the system is explicitly responsible for maintaining all material facts about its owner.
A mobile experience that truly represents your identity in a way that both resembles and enhances an in-person conversation but still affords you control over how you portion out your attention and provides context could tie the knot for the myriad communication channels available.
The first company to fully execute on embedding your identity into your phone (making a truly first class experience) wins the next decade.
R. Marie Cox (@artypapers) January 29, 2011
[Image via typedvorak.com.]
 The Facebook Messages product is arguably the first step in the first large scale attempt to consolidate all these communication channels and create a central stream for directed information. Your Facebook profile is also arguably the best representation of your public identity and how you want the world to view you in context of your peers. Those are two huge parts to executing on mobile identity and why rumors about a Facebook phone are so ripe. It's also possibly why the Messages product currently feels so awkward because it remains as an incomplete thought until the later stages of the product materialize. This is all speculation on my part, however; I have no inside information about a Facebook phone.
 Address Book, Twitter, Facebook, etc. all serve to fracture your identity. Android has an excessively aggressive address book that attempts to manage your social connections but provides no true representation of a person's identity within the system.
 If it were up to the carriers, we'd have identities composed of NASCAR ads and group buying coupons that require an extra monthly fee to connect to more than 10 people.
 Imagine priority calls, texts, inbox, feed, etc. And those are just the naive solutions, better ones are out there.