Why does my iPhone flash when I receive a message?

After the recent iOS upgrade, my iPhone flashes every time I receive a text message, and I can't find a way to turn it off. I checked the setting but there are no options where I can disable it from flashing. It is very annoying.

This is a feature introduced with the iOS 5 update.

Accessibility improvements:

• Option to light LED flash on incoming calls and alerts for iPhone 4S and iPhone 4
• Custom vibration patterns for incoming calls on iPhone
• New interface for using iOS with mobility-impairment input devices
• Option to speak a selection of text
• Custom element labeling for VoiceOver

You can change the setting in Settings -> General -> Accessibility options: