There are studies that show that reflectivity on the moving parts of your body is way more effective than a reflective jacket.
There was a guy riding the other night that had reflective material on the back of his calves and it was completely obvious from a long way away what I was looking at.
I was reading in a bike forum the other day that one guy bought two of those cheap lollypop reflectors that you use to delineate your lawn from the road, cut them down, and strapped them to the top of his rear stays, protruding about a foot on each side. He said he never felt such respect from the cars passing him. I wonder if the separation makes it easier for drivers to judge distance/speed/etc., if it's a depth perception thing.
That's pretty nice, but personally I think that money would be better spent on lights. A velcro reflective strap on each ankle and then kill 'em with candlepower.