I have always been well lit when cycling after dark but recently (October 2012) I have added flashing lights during daylight hours with stunning results.
My front light is a NightRider Newt single beam. My rear light is a Dinotti 300R - both have rechargeable batteries that run flashing for up to 12 hours. Both are extremely bright and cost upward of $200 each. Both lights are rugged and well made.
When I first began running the lights by day, both in flashing mode, I found it obvious that cars were seeing me and reacting to me sooner. Cars overtaking me changed lanes to avoid me at least 100 feet sooner on average, and cars ahead were much less likely to attempt a right hook or cross from a stop sign into my path. Maybe they think I am some sort of cop as both lights are brighter than anything most motorists see on bicycles. After two months I am completely convinced running with the lights by day make a huge difference in my safety.
I cannot run the front light flashing after dark as besides being bright and annoying to the point of rudeness I have actually had motorists ahead of me pull over to the curb as if being taken down by a police car or passed by an ambulance. Yes, it is THAT bright. By day it is more obvious that the blinking light is attached to a bicycle, not a public service vehicle.
I always wondered if the cost of such lights would be worth any added safety by day, and my answer is YES! Thought you folks would like to know this.