In the old days of iTunes 6 and earlier, the iTunes Source pane would contain a Library playlist that listed all the audio and video in your iTunes library. iTunes 7 removed this, and segmented the library into various 'master' libraries — Music, Movies, TV Shows, and so on. If you want to bring that overall Library playlist back, you can set two hidden preference keys in the Terminal:
$ defaults write com.apple.iTunes show-library-playlist -bool TRUE
$ defaults write com.apple.iTunes hide-library-playlist -bool FALSE
Ahhh – now I can see it all.