That's why I really like Chrome: it's all but impossible to use an old version without going way out of your way. If I give somebody Chrome, I know they're always going to have the very latest, standards-compliant version. As a web developer, you really only have to test the latest version of Chrome; there's no "Chrome 6", "Chrome 7", "Chrome 8" test lineup. Huge convenience to both parties.
Firefox is almost as good, but you have to upgrade between major versions yourself. The fact that extensions can break between versions really drags down the whole process, but there's nothing the Firefox developers can do about that. (they're at the mercy of the extension developers.) Pretty sure that's why Chrome's extensions are dumber, more lightweight -- Google didn't want a similar struggle.
IE is just terrible. Microsoft made upgrading such a milestone (long installation process, new GUI to adapt to, new intro wizards to complete), that it's totally natural that people cling to older versions. "The last time I upgraded Internet Explorer, everything changed and I didn't like it! I don't want the new one." And that's why you have to test IE9, IE8, and IE7. The fact that IE9 doesn't work on XP isn't going to help.
Unfortunately, I see the iPad as closer to the Microsoft camp (lots of discrete versions that have to be tested individually) than the Chrome camp.
While that's still true for iPad, it certainly is not true for iPod touches or iPhones. My 2nd gen iPod touch is stuck at iOS 3.3 with its completely different version of Safari.
And it's 2 years old.
So you're technically correct that Apple has not yet obsoleted the device they released one year ago. Given their track record though, I don't think I'd put too much faith in that continuing to be the case.
Lots and lots of users don't upgrade. I can't find it right now, but one iOS dev did a story on fragmentation in that camp based upon their own metrics, and it was scary. Perhaps this will resolve if they start doing OTA updates.
User "error" for sure, just putting that out there. Just because Apple makes updates available doesn't mean that most users have installed them.
I'm not sure if the bump numbers are broadly relevant: who is the average user who will be running bump? It isn't a general iPhone user, but instead is a technologically savvy individual usually in a place like San Francisco. There is no relevance there to the broader market.
Sidenote: This blog's highlight color is green on black? Ick.