Here in sweden by law all cars have their headlights on, day and night... In daylight you never even notice them, so I'm a bit skeptical about the effectiveness of bicycle lights in the day... certainly can't hurt though and blinking lights are possibly a bit better.
I would suggest that neon safety vests are more useful.

edit: actually the headlights do make a difference when seen in a rear mirror and make overtaking safer - especially for motorbikes.

OTOH at dusk you do start noticing car headlights much earlier than most people would consider it getting dark. I put my lights on when I start registering car headlights.