You must have updated to the beautiful new look of iTunes 11!
If you are one of the people who still want the sidebar, you can get it back easily by pressing Option+Command+S on your keyboard or selecting "Show Sidebar" under the View section of the menubar; for Windows users I believe the keyboard shortcut is Ctrl+Alt+S, but I could be mistaken.
The solution to this -- at least the one I know of -- is bizarre. Additionally, it doesn't disable launching iTunes when F7/F8/F9 are pressed, you need to do something different for that which I will mention after the main solution.
To stop iTunes from automatically launching when you plug in headphones or Bluetooth audio devices, go to "CDs and DVDs" in... (more)