I've only been able to invoke the feature by having a phone call come through when the phone is locked.
It flashes 4 times every couple seconds while the phone is ringing.
Straight from the Apple website.
"Accessibility
iOS 5 comes with innovative new features that make it easier for people with mobility, hearing, vision, and cognitive disabilities to get the most from their iOS devices. iOS 5 works with specialized hardware accessories. The LED flash and custom vibration settings let you see and feel when someones calling. And improvements to VoiceOver include custom element labeling. These are just a few of the ways iOS 5 continues making advances in the field of accessibility."