This topic usually comes up in the northern hemisphere autumn, just
before the end of Daylight Saving Time. That's when all the magazines
have their reviews of the season's crop of new products. Now in
summer you'll see off-season prices and thin selections at stores. In
most cases, it's worth waiting until the autumn product intros because
the LED emitters' semiconductor technology advances so rapidly.

Good references: http://peterwhitecycles.com/lightingsystems.htm and
http://sheldonbrown.com/commute

I've heard good things about the Planet Bike Superflash Stealth
taillight though I haven't seen one myself.