The iPhone's home screen works just fine with 16 application icons on the main screen and four more on the dock at the bottom. It still works well with another screenful of 16 more apps on the adjacent screen.
But, says Chris Devers, as you start adding more apps, the home screen UI doesn't scale well to cope with them. Flicking across five screens of apps to find the one you want is time-consuming. And moving an app from screen five to screen three is chaotic, unless you've left "gaps" on each screen as you went along - in which case you'd have six to juggle, not five. And even then, it's still chaotic.
OK, so not everyone is going to be collecting that many third party apps. But for people who do, says Chris, there needs to be a better solution than this. He's posted a set on Flickr to illustrate his point.
What might work? A Quicksilver- or Spotlight-style app, where you type some characters from the name of the app you want and it gets launched? Or a gesture launcher, where you "draw" what you're looking for?