Other than the iPod touch the iPods never had and never will have the iPhone OS let alone a newer version of it. To date I believe every iPhoneOS device can run the latest version of the OS. The difference between the 3GS and those that came before is the 3GS is considerably faster. There are other issues with missing hardware features on the older models as well, GPS, compass, video camera, iPhone is not immune to those little things.
I think that the problem with Android's apparent fragmentation is that there are so many different hardware options it feels fragmented. Throw in the actual differences, which on their own are minor, and the problem seems a lot worse than it is.
This is going to sound weird, but the main reason I haven't jumped into Android development is because there is no barrier for entry. Any loser can develop apps for android. That does two things in my mind. 1) floods the market with garbage 2) opens the door for malware, which we have already seen. Sure you can call me scared, but really I just want everyone to be held to the same standard as I am, and the standard I hold myself to is fairly high.... not as high as Apples standards, but much higher than the Android App store standards, which are none.