Michel, Android OS fragmentation mostly is an issue for apps and security. FWIW, Windows is *far* more fragmented than Android.
Focusing on apps: for Android, 97% of apps in the Android market are for 2.1 or later so you need to make sure your phone is 2.1 or later. IOW not much of an issue in real life.
Security: hasn't happened yet, but if a serious hole opens and your phone manufacturer shrugs, you've got trouble. I can't imagine that any of the big manufacturers would tolerate that, though.
I'm a great fan of Android. However, iOS is an excellent choice in October 2011. App store may be a PITA but it reduces the risk of malware almost to nothing. Then the walled garden philosophy makes it even safer. And all the apps are there.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
