Apple has released iOS 4.3, bringing improvements such as improved AirPlay performance, iPhone Personal Hotspot, iTunes Home Sharing, and, at long last, the option to configure the iPad's side switch to either lock screen rotation or mute audio. See "Apple Previews New Features in iOS 4.3 " (2 March 2011) for more details on what's included in the update.
iOS 4.3 runs on the iPad, iPhone 4 (but only the GSM model), iPhone 3GS, and the third- and fourth-generation iPod touch. To update your device, connect it to your computer, select it in the sidebar in iTunes, and click the Check for Update button.
An iOS 4.2 software update to the Apple TV is required for new AirPlay features to work, which is also now available. On the Apple TV, go to Settings > General > Update Software.