Here's a list of all the Android phones that Angry Birds isn't officially supported on, listed in a blog post by developer Rovio:
HTC Magic/Sapphire/Mytouch 3G
Sony Ericsson Xperia X10 mini
Rovio says that, despite having "hesitated to create multiple versions of Angry Birds" for Android, that's exactly what they're being forced to do in order to meet their goal of having it available on as many Android phones as possible. So they're creating a "lightweight solution" to run on devices that don't currently support Angry Birds, or run it kind of crappily.
Of course, you can see fragmentation in every platform—today's most awesome iPhone game, Rage HD, doesn't run on older iPhones and iPod touches. It's just more pointed with Android, because you're talking about more phones, and because some of the phones on that list, like the G2, are pretty current.
While differing system reqs is simply a fact of life with PCs—and no one would ever talk about it as "fragmentation"—it's definitely one aspect of phones turning into computers that we're not super keen on.
[Rovio via Twitter]