This thread got me thinking about aim on the backpack (which I thought was good). So I did a test... I had my wife stand behind me while I was on the bike in a normal cruising position. As expected the light was facing aimed toward the sky...but still visible from behind.

I rigged a clip on the bottom of the pack and attached the blinky to that. A-OK!

While I was at it (and in response to the DE laws on biking...see "Ticket This Morning Thread") I got a set of thse arm bands for even more strobe goodness

Planet Bike BRT Strap