Twice in the last week I failed to receive text messages. The cause: Apple’s iMessage. I activated iMessage when upgrading my 3GS to iOS 5. In doing so, Apple took note of my mobile number and now, if another iMessage user tries to text me on that number, the message is automatically sent over an internet connection within Apple’s proprietary messaging architecture instead of via SMS, regardless of whether my SIM remains in the iPhone or not.

As a result, the iMessage users trying to contact me were, in fact, only contacting my iPhone – which was sitting in a speaker dock at home. My SIM, as is often the case, was in a different handset, so while I was travelling around, texts from iMessage users were going unnoticed on my iPhone.

In Apple’s iMessage system, your identity is now your device, not your SIM card.

It appears seamless to the user and, in many ways, is a benchmark example of Apple’s approach to user experience. The underlying transport mechanism is irrelevant to the user: they simply want to send a message and Apple sees its job as ensuring it reaches the recipient in the easiest way. There are also attractive side effects: no SMS fees and no extra charges for sending multimedia.

However, it highlights 2 significant changes:

1. Your SIM is no longer the ultimate arbiter of your mobile identity. Previously, you could virtually guarantee that text messages and voice calls would reach you on any handset, anywhere in the world, simply by inserting your SIM card.
2. Your network operator no longer knows you best. If you are an iOS user, it is Apple who has the most complete picture of your behaviour; for Android users it will be Google. For any operator still harbouring ambitions of generating new value from its knowledge of user activity, this is your wake up call: the old guarantees – the SIM and the mobile number – are no longer worth what they were.

Of course, I am unusual in my mobile habits. In practice it is a relatively small percentage of iPhone users who swap their SIMs into different devices, but the market is now large enough that even a few percent represents many millions of people and missed messages.

Why not take the approach adopted by the likes of WhatsApp – a free messaging application which pre-dates iMessage – where messages are only routed over the internet when they know your SIM is active in the current device?

There is also a wider question here, one explored by Ramona Liberoff in her session at the May 2011 MEX event. How do we tell the digital environment who we are in an architecture of multiple digital touchpoints? The day when every user is engaging with the digital medium through multiple devices is not far off. How can we translate something as complex as human identity into a language this digital world understands? Will it be possible to select which parts of our identity we wish to reveal in specific parts of this digital architecture? Will we be able to represent ourselves though something more human than a SIM, a mobile number of a username and password?

Who we are and how we present ourselves is so much more subtle than the binary logins we use today. More thought should be given to reflecting the complexity of the handshake, the kissed cheek, the wave, the hug and business card in digital language.

P.S. If you’re experiencing the same problem using iMessage with multiple devices, the trick is to change the default setting so texts from other iMessage users are still sent over SMS rather than via Apple’s architecture.