You can quickly mute any sound coming out of the iPad speaker with one button. Simply press and hold the volume down button and the sound will turn off.
To bring the volume back up just press the volume up button to increase the sound.
UPDATE: You can also mute sound on the iPad with the switch above the volume buttons on the side of the iPad. This switch was previously the rotation lock switch. iPads running iOS 4.2 now have rotation lock in the multitasking dock.
UPDATE: iOS 4.3 makes it possible for iPad owners to change Settings and bring back the rotation lock switch. Now users can decide if they prefer the switch as a mute or rotation lock function.