Shawn Blanc has a good piece about what iOS adoption rates could look like come this fall:

Note that there are two devices on Dempsey’s chart that can currently run iOS 6 but won’t be able to run iOS 7: the iPhone 3GS and the 4th-generation iPod touch.

The question is: of the 93-percent of active iOS users who are on iOS 6, how many of them are using a 4th-generation iPod touch or iPhone 3GS, and thus won’t be able to update to iOS 7 this fall?

It’s certainly not a majority, but also likely non-trivial.

I agree with Shawn here. It’ll be interesting to see what happens. I can only speak for my household and say that of the 5 iOS devices we own, only 3 are eligible to be upgraded. While I don’t know if my 60% translates to the larger group of millions of iOS users, I do think that it will be a large minority. The 4th generation iPod touch was a huge hit, particularly with kids. My 10 year old is a perfect example. He’s a geek like me, and is totally aware of what iOS 7 will (tentatively) offer, but isn’t interested in buying a 5th generation touch mostly because he says he loves the size and doesn’t prefer the 5th gen which my older son has.¹

I think developers are in for some growing pains when iOS goes public, and we all know they’ll be able to handle it gracefully, but I really do hope this isn’t the beginning of a fragmentation problem like another certain mobile platform.