Its not - You couldn't even get search and scrolling-overlay onto the 5G...
I think, iirc, that they put the scroll overlay on the 5.0G, but not the search.
And it would be great if they could put CoverFlow and whatnot onto the 5.5G, but I wouldn't expect it to be possible. At the very least, its a new feature that Apple wants to keep to its new model, to ensure that people will buy it simply for the new features. More likely, it uses some new piece of hardware inside the iPod to optimize it, and therefore is a complete impossibility.