How to quickly mute sound?

Push and 'hold' the down volume switch on the side. If it is held, it goes to mute immediately.

Cheers.

Wasn't the rotate lock switch originally intended for muting? I wonder if there's a pref or a way to get it back to that.

No, it's hardwired. It was a good decision, though, I feel because locking the screen orientation easily is mighty useful in the iPad. Wish the iPhone could do it. The compensation for the dedicated mute switch works almost as well, I think. After less than a second of holding down the volume toggle in the lower volume direction, the thing mutes. Almost the same as clicking the iPhone's mute button, no? I also find the iPad's volume toggle switch is easier to use than the iPhone's.