While I understand what you're getting at, the situation is a bit different for Android. If you have an Android device that is a few months old when a new version of the OS is released, you will probably have to wait several months for an update (if it comes at all). It isn't exactly uncommon for Android devices to ship with a release of the OS that is older than what was available at the time of the devices' release either, and those devices are unlikely to receive an update.

That said, one of the goals of Android 4 is to separate a lot of the components from the core OS so that updates can be issued independent of the device developer. It doesn't solve the issue of fragmentation in the core OS, but it does reduce the fragmentation with many of the applications and libraries that ship with Android.

(Another that said: fragmentation is actually a benefit of Android. It has allowed Android to ship on many devices that it wouldn't have if tight control was exerted over the OS. This ranges from project boards for hobby electronics to consumer oriented game consoles.)