One that most users will appreciate is how the new Notifications feature displays updates on the iPhone's Lock Screen. With the new system, you can decide how obtrusive you want alerts to be. New notifications come in two flavors: the typical pop-up messages that need to be dismissed, and less distracting messages that unfold from the menu bar, automatically displaying and dismissing themselves without interrupting what you're doing. Any missed alerts can be accessed by swiping down from the top of the iPhone screen to reveal the new Notification Center.
Alert options can be modified on a per-app basis under Settings > Notifications. Here you can decide whether the Notification Center alerts are sorted manually or by time received; whether they display in the Notification Center; how many alerts can be displayed per app; the alert style; and whether they show up on the iPhone's Lock Screen.
For that last option, the Lock Screen has been modified to accept and display alert notifications. A swipe of the app icon in the Lock Screen alert takes you directly to the message itself in that particular app. The ability to quickly access exactly what you want from the Lock Screen is a well-implemented time-saver.
In what I suspect is an effort to enrage mobile carriers profiting from overpriced text (SMS) and multimedia (MMS) message plans, Apple has included a new app, iMessage, as part of the core software. Just as with SMS and MMS, communication through iMessage relies on text, and allows you to send movies and pictures -- without being charged for doing so. iMessage circumvents the carriers entirely, instead relying on Apple's own servers to handle the encrypted communications.
SMS and MMS messages are still available in the iMessage app; but if you are communicating with an iPhone, an iPod Touch or an iPad running iOS 5, iMessage picks up on that and uses Apple's servers to deliver the messages. When that happens, bits of the iMessage interface like the Send button and chat bubbles turn blue; when sending SMS/MMS messages, the interface is green.
With iMessage, you can see "read" receipts -- which notify you when a recipient has read the message -- take advantage of group messaging, and associate an iMessage account with multiple phone numbers and email addresses. iMessages are delivered to all devices with the associated AppleID.
I've noticed that sometimes delivery of an iMessage is delayed, especially on the iPad. (If the iPad is asleep, it won't wake up to receive the message like the iPhone does, but it will receive the message if it's awake.) If an iMessage can't be sent, it will come in as a regular SMS/MMS message -- this behavior can be toggled on and off under Settings>Messages. However, if your family uses iPhones, this will be a money-saver. Keep track of your messages pre- and post-iOS 5 update to see if you can cut back on what you pay for texting.
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