A misconception - 1 SIM = 1 Identified User

There seems to be a common belief that because all GSM/3G phones have SIMs, and that everyone has a phone, this means that the SIM is an ideal platform for determining identity. I've heard people suggesting integrating SIMs with healthcare records, payment cards, ID cards and other similar alternatives.

I'd urge caution on this. The world is not as simple as you think.

Some of the problems:

• People have multiple SIMs, and multiple devices. While this doesn't apply to everyone, a growing fraction of individual users will fall into this category. I'd expect far more people to have 2 SIMs than 2 nationalities/passports.
• A few people have no phone/SIM and no inclination to change. Some form of 'digital inclusion' policy would be needed for the cellular refuseniks.
• *EDIT* - also see my more recent post where I try to quantify the two factors above
  
• Many countries still have anonymous prepaid SIMs. While some countries like Italy & Thailand force SIM purchasers to register their ID, that's a far from uniform situation. I've bought loads of SIMs over the years with total anonymity. And I'll bet that black markets exist even in countries where registration is mandatory.
• Just because a SIM is registered to an individual, that doesn't mean they're the user. A speaker from India yesterday said that SIMs are usually registered to a head-of-household, rather than the actual user.
• No idea what happens with legal responsibility & identity with children with SIM cards, but I'll bet it's complex and variable per-country
• Plenty of SIMs are used by machines, not people. How do you know if it's actually a vending machine that's sending a message, or someone hooked up via a PC?
• Some phones or other devices are shared between multiple people (eg in 'pooled' devices in businesses, cellular fixed phones, or phones for a whole village in emerging markets)
• There is some evidence that young people swap phones (& SIMs) amongst each other in social groups. This could be because of the dynamics of who has prepay credit among friends, for example, or in situations where an operator doesn't offer x-network minutes and it makes sense to use someone else's phone to save money.

The last point is perhaps the most crucial. The whole idea of one SIM = one person it hugely exposed to the risk of social dynamics and fashions. It only needs one group of teenagers to come up with a 'killer app' for phone-sharing or swapping (maybe it becomes cool to have a new number every week), you get viral adoption around the world in a couple of months, and the whole identity/SIM linkage falls apart.

Ironically, the use of Internet access on mobile devices exacerbates this risk, as potentially users can sign into Yahoo/Google/Skype/Truphone/Fring from any device rather than their 'official' phone & SIM.