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While there are no specific details that Apple will provide to developers that is an easy-to-use chart with live updates, the adoption rate is about 90% for each new iOS within six months. The latest now, of course, its iOS 5, so most likely, 90% are on iOS 5 now with a very small percentage on iOS 4, and almost no one on iOS 3.
Here's how the adoption rate normally looks:
1 month after release - 50% adoption rate
3 months after release - 70% adoption rate
6 months after release - 90% adoption rate
Here's a cool graph that shows that it's always around 2% of users that are on 3 (compiled using a number of reputable sources):
I would consider that statistically insignificant.