The Slashdot: Apple Releases Major iTunes Update discussion includes the following gems (unattributed, because I'm lazy):
Lastly, and this is a feature of the entire music library, not just playlists or Party Shuffle, the same "arrow" icons that show up in the iTMS when you search for a song are present in iTunes. This means you can click an arrow for a song name, album, or artist and it will launch a search on iTMS. But say you don't like that feature? Well you can of course turn it off in preferences, but you may also hodl down "option" and click it. The result? it searches only YOUR library, not the iTMS.
You can now use other playlists as criteria for a Smart Playlist. Create one playlist that is a combination of several other playlists.
