The only relationship between the version of the OS and fragmentation is that a new OS rev usually introduces an additional API level. So if you write an app to API level 15 (4.0 IIRC) that will be available and work on subsequent revs.
I think you're missing the point. Putting it another way: When the 4.1 APIs are released, how long do you need to wait to add 4.1 as a dependency for the core cool new feature in your app? Obviously you can use the 4.0 APIs that are still there, but you have to admit that your ability to push new features to your whole user base is impeded by the rag-tag update schedule.
Yes I agree that you cannot target the the latest API, and there is a lag, which has been exceptionally long because of the Honeycomb blip.
But it just means that, form factor and to a certain extent performance differences are more likely to be a problem than perceived platform level fragmentation IMO.
