The only problem is there's few dual SIM phone options on the market so far, all Chinese. And dual-SIM backs where you have to reboot the phone to change SIM are not going to cut it.

There's probably more chance of a snowy day in hell than a major handset manufacturer producing a proper dual SIM handset, seeing as they depend on the network operators (who hate people who SIM-surf)...

They're overlooking the fact that many people purchase pre-pay SIMs and then dump them for various reasons. The most common being when you're visiting another country and don't want to pay international roaming rates.

If I'm visiting anywhere in Europe for more than a few days, it's always worth buying a local prepay SIM. The cost of a SIM (usually about €20 ish including credit) is far lower than paying through the nose for international roaming calls on my own Irish mobile.

I bet those rarely used / abandoned SIMs make up for a huge number of accounts that bring the populations over 100%.

There could be quite a large number of SIM cards in use by tourists, visitors, business people and short term residents in any country.