I have the light you are talking about, but I can't remember the brand. It cost something like $24 atthe LBS. I have only seen the light at one shop. Planet Bike does sound right, though.

It is ridiculously bright and I often feel bad about using it, but you can see it extremely well during the day, which is when I ride. My wife's typical Target blinky can't be seen at all during the day, and it's pretty much useless (again, as we only ride during the day).

I know it is annoying to look at, but the point is that if you annoy a driver because of your rear blinky, then they definitely saw you in the first place, which is the point. Also, because of the light, I feel I get a lot of respect from motorists. I have over 550 miles logged in a big city just this season, and not once have I been harrassed or honked at (although my wife has--once they found out she was with me, they scurried away in their huge 24-foot dock truck).

If I am doing a mixture of road and MUPs, I try to remember to turn off the blinky on the MUP, but I do forget to turn it off more often than not.