I just discovered that the latest iPod updater (2006-01-10) fixes an issue that's bugged me for quite awhile. It's even the covered in a Knowledge Base article on Apple's website (which now refers to the latest update as a fix).
You may or may not know it, but at some point, the iPod software stopped working correctly with "dynamic" Smart playlists on fourth generation iPods. For instance, I have several playlists that have a criteria for omitting songs that have been heard in the last two weeks. On the iPod, even after a song has played, that playlist wouldn't recognize it and omit the song.
Well, the new updater fixes it, to a degree. The only way to get your dynamic playlists to update on your iPod is to navigate backwards to the playlist level, and then start playing a different playlist. Then, when you switch back, the dynamic list will be updated. I believe that this is the only way you've ever been able to get any sort of dynamic playlist to update on an iPod. I know it did work previously, because I discovered the 'switch playlists to update' trick sometime early last year.
By the way, this issue doesn't affect nanos or 5th generation iPods.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006011315403811