Here are some more ideas:
You could sew some SOLAS reflective tape
onto it to add to the reflective surface.
There are also lighted
reflective bands like NiteIze
stuff, which give off their own light so they show even if peoples' lights are off, or people who don't have lights (ninja cyclists, pedestrians). And they reflect when there's incoming light to reflect, of course.
I have a PolyBrite self-illuminating belt that is pretty visible at close range. http://www.hivis.biz/products/safety...1&tid=5&pid=95
For $5.50, hey.
If you ride fairly low (like on a road bike) then you could "U" this around the bottom & sides of the backpack to get rear & side visibility, I guess you'd want to stitch it on. If you ride more upright (hybrid or mountain bike) then it might be better just to wear the belt around your waist, fairly low across your behind.
And of course a person could clip a couple small blinkies onto the bag, all the reflectivity in the world is great but active lights are still valuable. I was riding on 29th here, they've just repaved it and they have a bunch of traffic markers. Well, it was most of the way to nightfall, and I had my HID headlight on, and it was surprising to have road-construction workers appearing out of nowhere, with nothing but reflective ANSI vests (orange). Their reflective tape simply blended into the surroundings of those traffic markers. If their vests were neon-lime instead of orange, that would've helped, but just one econo-blinkie would've done everything to make them stand out.