The data seems pretty clear. Prior to June 8th we have a fairly low adoption rate of ~3%. Starting on June 9 this jumps up to 6-8%, which can be directly tracked to the developer release at WWDC. Starting on June 17th we get a huge jump as all the non devs start upgrading. We’re currently running at an overall 75% upgrade rate which is pretty insane considering the number of devices and the fact that its only been 5 days.
The article is a great read both because this level of uptake in an upgrade release is phenomenal—even with the iPod touch, which is around 50% (and you have to pay $10 for it)—and because the statistics and data in the post are, well, really nerdy.
Apple deserves credit for making the upgrade process more or less an automatic thing. You sync the device, it checks for updated software, and shouts at you to upgrade. A couple of clicks and some time later, and you’re good to go. Lots of things should be so easy and decided for you.