Update: This doesn't work on newer versions of iOS.
Sometimes, I want to hear a song on YouTube while doing other stuff on my iPhone. After a bit of poking around, I figured out how to do it:
- Start the video playing.
- Hit the Home button to return to the home screen.
- Double-tap the Home button to bring up the multitasking tray.
- Swipe to the left and hit the Play button.