If you've been waiting for months for Apple to release the iOS 4.2 update, it's here. It's also an update for all other supported iPhones and iPod touches. The update doesn't support the original generation iPhone, or the first generation iPod touch; those are both at the end of their update life cycle. Some of the features in the iOS 4.2 update won't work on the iPhone 3G or the second generation iPod Touch.
To update a compatible device, you need to connect it to your computer, launch iTunes, back up the device, then click the Upgrade button, and wait, making sure you don't dismount the device until the update is complete; be prepared for it to take quite a while, and for the device in question to reboot.
This update includes AirPlay, the replacement and improved version of AirTunes. AirPlay lets iTunes and iOS devices stream video to a TV connected to the new Apple TV. What people aren't pointing out, maybe because it seems to obvious, is that if you've got an AirPort Express connected to a stereo, you can stream music over WiFi to your stereo from an iPad or iPod Touch. AirPrint is sort of, included, but it's not the version and feature set of AirPrint that was initially promised. You can't use a PC or Mac as an intermediary in order to print, at least not in this version of iOS, but you can print to an "AirPrint enabled printer." That's a pretty small list right now; let's hope it improves.
Apple is also now supporting, at no charge, the Find My Phone feature for owners of iPads, iPhone 4s and 4th generation iPod Touches. That's the service that uses an Apple ID, and a MobileMe to help you locate a missing device. Now, if you own one of the supported devices, you can us a free MobileMe account to activate the Find My iPhone services. You can track the device on a map, display a message asking someone to contact you, or have it play a tone so you can find your iPhone behind the couch cushions. You can also remotely wipe the device (or lock it), and thereby protect sensitive data.
Those of us with iPads have been waiting, with various degrees of impatience, for this update, because it supports multitasking, of a sort, and "folders," for organizing apps and Web links. Now, apps can perform some functions in the background, with another app in the foreground having "focus." Apps written to be compatible with iOS 4.2 will now save their state and "remember" what they were doing when you left them, because of the new "fast app switcher," engaged when you double-tap the Home button. So, let's say you want to listen to music while you read a book in iBooks; launch the iPod app, start some music, then double-click the Home button; a new dashboard panel slides open at the bottom of the iPad, and you can launch iBooks, and read while you listen. Keep in mind that not all apps support this yet, but I'm already already finding it useful to move quickly between Mail and Twitter as I write posts, or listen to streaming music via Pandora.
Othere updates include customizable identification tones for text alerts on iPhones, calendar support for invitations from Yahoo!, Google and Exchange calendars, a unified inbox and the ability to organize email by conversations/threads in Mail, fonts in Notes, and the much vaunted Game Center, as well as a fair number of security updates in iOS 4.2.