Apple, on the other hand, dodge the issue of fragmentation completely. iOS is a completely unified platform, with only one catch: you must be up-to-date with the latest shiny toys. The new iOS 4.2 is now available, with the following small caveat:
The iOS 4.2 update is available today to download to iPad, iPhone and iPod touch by syncing the device with iTunes 10.1. iOS 4.2 is compatible with iPad, iPhone 3G, iPhone 3GS, iPhone 4, second and third generation iPod touch (late 2009 models with 32GB or 64GB) and new iPod touch. Some features may not be available on all products. For example, Multitasking requires iPhone 3GS, iPhone 4, third generation iPod touch (late 2009 models with 32GB or 64GB) or later.
So, as with recent OS updates, it won't work with my first generation iPod Touch (or second gens, for that matter). Owners of those devices are left with older versions of iOS, with no upgrade path that doesn't require a hardware purchase.
Meaning that, if you're an application developer looking to target the broadest user base, you're hit with platform fragmentation...