The iPhone 4S and new (third generation) iPad are the only two devices guaranteed to get every single benefit of iOS 6 this fall. (And that's assuming Apple doesn't add some cool new feature that only works on the iPhone 5 later this year.) Every other iDevice will have a watered-down version of iOS 6.
By the way, the original iPad is stuck on iOS 5 forever.
Now, is this fragmentation? Technically, yes.
But it's still nowhere near the level of fragmentation you see on Android devices.
Even if you have an iPhone 3GS, the oldest iPhone model still being sold, the core iOS experience will remain the same. All your apps will work. You'll still have iCloud. And you'll still get some great iOS 6 features like Facebook integration and the new App Store.
Compare that to Android phones. With so many forked versions of Android, it's still super tough for developers to make sure their apps work on every one of them. Just try using Hulu Plus on a new Android phone and you'll see what I mean.
However, I don't think everything Apple is doing here is perfect.
So why is Apple blocking this stuff on older phones?
I was happy to hear older iPhones like the 3GS would support iOS 6. But after looking through all the details, I'm not so sure Apple is doing the best thing for customers who are attracted to the lower-priced iPhone models like the iPhone 4 and 3GS.