What are we talking about
Linux Fragmentation. In fact it's the stack above the linux kernel which lead to fragmentation. You have Openmoko, QTopia, A La Mobile, ALP, Maemo, Moblin to name very few of them. They are more likely incompatible between them. some rely on KDE, some on GTK, some on other FW. and it's similar on the desktop environment.
Even Nokia it at the head of two incompatible environment Maemo for the table and QT/QTopia.
Here I just browse the UI aspect. I'm not talking about the rest of the frameworks. The data connection access usually rely on the existing standard UNIX/Linux stuff. But a soon as you are touching phone specifics, you have the choice across variety of stuff. ALP, QTopia, OpenMoko have different ways of handling the SMS, phone call and so one.
If this is not looking like fragmentation, what is fragmentation then?
All can be said on Symbian, but it provide a kernel AND a complete set of frameworks for all the required features around a phone. on to of this the UI is specialized, some feature are added by the UI provider or the handset maker.
Even Java on mobile suffer from fragmentation. How many complains from developers who have an application not running the same on two devices from the same handset vendor.