Oh definitely it's a good adoption rate. I just think a lot of people were thinking "60% in a week! We'll be at 90% in a month!" and that seems overly optimistic at this point. And if it plateaus at 70%, how many developers are willing to cut off 30% of their customer base?
History has shown that won't be the case. We might not be at 90% by the end of the month but mark my words, the exponential growth of iPhone sales in which each new model eclipses the entirety of iPhone sales up to that point has a pretty drastic effect on the old OS percentages.
The combination of the iPad, iPad mini, iPhone 5, and new iPod touch all running iOS 6 and the aggressive older device update strategy means we are pretty much assured 90% by the end of this year, if not sooner. With the notable exception of the original iPad, the iOS devices that can't run/won't run iOS 6 are a drop in the bucket.
The other thing to note is that as devices get older, users tend to enjoy them less and therefore screw around on them less. This naturally results in less app usage, less browser usage, appearing in less statistics, etc...
So while the exact percentage of all iOS devices running iOS 6 might take longer than a year to hit 90%, I suspect app developers will have a fairly different picture painted for them by their personal usage statistics.
