Apple uses new features and UI enhancements that are only available in the current generation of iPods as a carrot to get people to upgrade to the newest model. The new UI on the iPod Classics is a great example. These features could have been placed on the previous generation of iPods with a firmware update, but Apple decided not to do that. as they have for most of the iPod's history (which is their right, of course).
Some eager hackers have modded the 5th gen iPod's firmware to make it look more like the iPod Classic's. If you're jonsing for split screen navigation but don't want to pony up for cash for a new iPod this is for you.
Keep in mind, though, that this entails changing the firmware that is currently on your iPod (which is Apple approved, the firmware that is, not replacing it) to something that was modified by a third party. Bad things could happen (though they could just as equally not ) so be prepared to deal with the consequences should the process run amok.
Thanks, Kai Cherry.