We have to factor in that the sales number counts the original model as well.
In the US, at least, quite a few of the 3G sales were to original owners. In other words, quite a lot of sales were to the same person.
So the percentage by population is less, perhaps much less, than figured above... though still respectable.
I have a 1st gen iPhone that a friend gave me when he upgraded to the 3G model last summer. It is unlikely that many people put their old iPhones in a desk drawer when they upgraded, and O2 offer a pay as you go sim-only pack for such a circumstance.
There are as of now over 1 million iPhones that have been sold in the UK, and apart from a few broken ones, I would expect almost every single one to be in service as of now.
UK phone suppliers have been moving away from a 12 month renewal cycle to an 18 or 24 month cycle over the past two years to help offset the subsidies they are having to increase on handsets, so the iPhone in the UK has another 3 months to go before it has been available for what is now 1 whole contract cycle (18 months).
Many of those 70 million phones that are said to be 'in use' are cheap and cheerful pay as you go emergency-only phones for kids or older people, many more are business mobiles provided by companies with little or no choice for the end user.
Put in this context, the depth of penetration of the iPhone in the UK is phenomenal. I have many friends who are Mac sceptics who have now got iPhones, and without exception they all agree that it is the single best tech purchase they have ever made...
...and they are telling their friends. The word of mouth on this product is unlike anything I have ever seen in the tech world, it literally sells itself.