I've had a Catlike Urban helmet for a couple of years now with a built in blinkie/steady light (see below).
Can't say it's very bright, but I've been told by other cyclists that it did get their attention, so I believe that cars should notice it too.
Although I would seriously advise on having AT LEAST one more light since it's relatively easy to accidentally turn it off; I ride with two additional lights, a steady dyno driven light built into the rear rack, and a 1/2 watt flashing hi-powered unit just below the seat.
