Google recently announced that the Nexus One phone will not be upgraded to Android 4.0 Ice Cream Sandwich.
There have only been 2 flagship Google phones which fall under the trademark Nexus released to the public.
From Wikipedia, the first Nexus phone, the Nexus One was released:
Gabe Cohen from Google said that Android 4.0 Ice Cream Sandwich is "theoretically compatible":http://www.engadget.com/2011/10/19/google-confirms-nexus-s-will-get-ice-cream-sandwich-for-real/ with any Android 2.3.x device currently in production.
You would think that by buying a less than 2 year old phone in the Nexus One you would be somewhat future proof. And technically yes, you could load a 3rd party ROM but that's not the point. The average person isn't going to root their phone and go through forum instructions.
The point is, Google officially does not care.
Android fans may deny it but the Nexus phones represent the best case scenario in terms of receiving updates as they are officially endorsed by Google. But time will show that of the dozens upon dozens of Android phones currently in use, most will not see carriers/manufacturers push out a version of Ice Cream Sandwich to their phone.
This model of 'just buy a new phone' may work some what with mobile phones where contracts are largely on a 2-year cycle but depending on where you fall on the purchase cycle, customers still fell burned (in this case your technical users will notice).
The model of poor upgrade management works very poorly with tablets as users expect a longer life span. So when factoring in software upgrades, don't forget the dozens of tablets which run Android 2.x.
The situation of fragmentation may improve with the next release but in the meanwhile you burn a lifecycle of users who bought your product that at the end of the day didn't have a very long life span.