published a nice chart of what iOS version people are using.
As part of the WWDC keynote, Apple has also said they've sold 600 Million iOS devices. 1% of that is 6 Million.
But people have broken them (which knock-on-wood I've not done yet), replaced by a new device (which I've done a LOT.) Lets say 2/3 of the devices that have been sold are still active.
So 1% of that 400 Million actives is 4 Million who are using an iOS 4.x or older version.
Apple posts there are 275,000 registered iOS developers in the US. It's easy to project that between registered developers worldwide and support teams (testers, designers, etc) there would be well over 1 Million of them. Maybe even 2 million. [UPDATE: in the recent Q3 earnings, Apple said there are about 500,000 developers in China, so that should make way over a million developers worldwide--so could easily be 3 or 4 million developers + support teams.]
If they're like us, they each have an old device or two they keep around for testing apps...I've got an old iPod touch and a first generation iPad.
What if that the majority of that 1% using old iOS versions is all the developers testing their apps on old iOS versions!
We're going to require iOS 6 on the next update to our biggest app, FTP On The Go Pro.