iTunes 11 - View Playlist and Library at same time...
(I have not heard of this but found it the other day...)
View a playlist in a separate pane on the right side of iTunes music.
Turn off iTunes sidebar. (iTunes menu View > Hide sidebar)
At top of iTunes, select Playlists.
Now you see your playlists on the left side of iTunes.
Select one (not a smart playlist), right click > Edit playlist.
You will now see the playlist on the side and the Music on the left.
You can still view music by Songs, Albums, Artist, Composers, Genre and drag songs to the right into the playlist.