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.