Comparing it to PCs doesn't make sense because you always have the option to upgrade your OS
The flaw in that logic is the user has to BUY the new version of windows, and most don't (until it's pre-installed on a new PC). There are at least 3-4 common versions of windows still running out there, all the way back to Win2000.
it's not a flaw in the logic at all. if the user had the choice to buy an android upgrade there wouldn't be talk of fragmentation and devs would be able to create apps specifically for the OS version they wanted to and not have to support older versions (you bought an OS upgrade at a premium so you get to use my app). the fact that these os revisions are offered for free by google, but then never see the light of day on the 60+ android devices proves that fragmentation is a real problem. and thats because google has no standard. they just release a crapload of OS versions, begin to ignore the old ones, and fail to realize that carriers and manufacturers are not passing the OS's along to the end user. So the dev who's developing for Gingerbread ends up supporting phones running Android 2.2 because it'll be available in their Market and then force close twenty times.
if MS OS updates were free and were dependent on the manufacturer we'd have the same problem. If HP releases updates and all their computers are running Windows 7, whereas Dell is still on Windows Millenium edition, you think people wouldn't complain about fragmentation? Especially if there was a "Windows Store" where everyone can download software? The reason you don't hear about PC fragmentation (except when you're defragging your hard drive :icon_ banana is because the user has the option to upgrade. Whether you pay for it or not is irrelevant.