Heck, apps are badly broken within the SAME version of Android, let alone differing. I kindly invite Mr. Schmidt to sit down with me as I browse the Android Market. I have a Bionic, and a majority of EA and Gameloft's top titles are "not compatible with your device". I have a modern device, running a modern OS. If fragmentation isn't there, and isn't breaking apps; then why aren't apps simply compatible as long as the device has enough memory and processor power?
[if there were fragmentation then apps would be far more broken across the differing versions of Android, and different third party skins of Android