iTunes 4.7.1 (the new version that you need for an iPod shuffle) includes a new option labeled "Keep this iPod in the source list". What's up with that? When you check this box, and then unplug the iPod shuffle, the iPod doesn't disappear from the iTunes source list, as ejected iPods always have before. It sticks around whether the iPod is there or not.
What is this phantom iPod good for? Even though the iPod isn't there, you can mess around with its music library: add or delete songs, create a new list with Autofill, even change the song order. The next time you connect the iPod, it will update to reflect the changes you made. Handy! One of the iPod folks I talked to at Macworld called this "shadow mode".
One problem: the iPod icon in the source list looks exactly the same to me whether the iPod is connected or not. If there is a difference, it's too subtle to see. The icon should turn gray or fade in some way to show that there's not really an iPod there.
In iTunes 4.7.1, shadow mode works only with iPod shuffle, not with any hard disk iPods. Seems like it could be useful for big iPods too.
UPDATE: A friend pointed out that we already have something like this for hard disk iPods: playlists. Unless your hard disk iPod is in manual update mode, it's set to sync with playlists in your iTunes library. You're free to change those playlists even when the iPod is not connected, and all changes flow to the iPod when you sync. You can even think of shadow mode as a substitute for this feature, because the iPod shuffle has no playlists.