iPad Support Group

The button just above the volume bar is the mute button.  (A word to the wise, make sure you mute your iPad before opening something like Angry Birds during a meeting...just saying.)

If you are using your iPad in a situation where you do not want the screen to change orientation (for instance, if you are connected to a projector, or reading a book on it while laying down), the mute button can be set to be the Screen Rotation Lock instead in your System Preferences.