Apple's latest update to your iDevice brings personal hot spots (for the iPhone 4), better AirPlay, improved Safari performance and allowing you to use the iPad's side switch as it was originally meant to be - an orientation lock.
apple.com - Download the free iOS 4.3 Software Update and get new features that let you do even more with your iPhone, iPad, and iPod touch. The software update is free. It’s easy. And you can download it right now.
More info about iOS 4.3 here.