iOS 5 tip: How to enable LED flash alerts
iPhone 4 and 4S owners can use the LED flash on the back of their phone to alert them of incoming calls and notifications.
iOS 5 introduced a number of new and varied notifications, from the Notification Center and different alert tones for different apps to custom vibration patterns and a new visual alert.
Buried in the accessibility settings is a new feature that uses the LED flash next to the rear-facing camera on the iPhone 4 and iPhone 4S to notify you to incoming calls and alerts. It's a great feature for the hearing impaired and anyone who would rather receive visual rather than audio alerts.
To set it up, to to Settings > General > Accessibility and then scroll down until you see LED Flash for Alerts. Turn the slider to On. For the LED flash feature to work, you will need to put your phone in silent mode by turning off vibration in Settings > Sounds and also flipping the switch on the side of your phone to turn off the ringer. You'll also need to get in the habit of resting your phone screen down (though when face up, the screen will turn on when a new notification arrives). Now, when you get a call or a new notification, the (very bright) LED on the back of your phone will blink three times.
For more tips, check out the complete guide to iOS 5.