This is a really big deal, because if you previously wanted your app to feel like a native Xoom app, you had to write off the Samsung tablets and all phones. But now that the fragment API has been backported, it's possible to work well on the newest tablets and gracefully degrade on older devices and phones.
Android got screwed big time by the carriers, and it looks like Google is now trying to make up for that, by exposing new features through libraries. Too bad it has come to this ...
Considering that many developers have suggested that the solution to OEM/MOs not updating Android devices fast enough would be to back port stuff..its a welcome sing from Google.