Fragmentation is really variable depending on the kind of app being made. In terms of UI design, fragmentation is not much of an issue as there are a lot of techniques and libraries, official and unofficial, to allow UI patterns across 90%+ of devices.
The real issue for fragmentation comes from manufacturers and carriers changing the OS for their purposes in a way that breaks some API functionality in some or all cases. Apps that have to do tricky things with streaming data, multimedia playback, and hardware sensor interaction have this the worst, but just how bad it will be really depends on what you are doing. It should be noted that most devs will never have a major fragmentation problem and that the platform is improving with each major release to make these problems less apparent.
The real issue for fragmentation comes from manufacturers and carriers changing the OS for their purposes in a way that breaks some API functionality in some or all cases. Apps that have to do tricky things with streaming data, multimedia playback, and hardware sensor interaction have this the worst, but just how bad it will be really depends on what you are doing. It should be noted that most devs will never have a major fragmentation problem and that the platform is improving with each major release to make these problems less apparent.