Well... I say Brain Bag, but I think adding retroreflective material like 3M's Scotchlite would be an excellent option for any of the bags, especially those that are used by folks on wheels.
I recently got a motorcycle (when gas went past $3). I love my Brain Bag, and with the waist and chest straps, it makes an excellent bag for a bike. Once the straps are all adjusted, it's incredibly stable and doesn't shift around which can be quite disconcerting when cornering or traveling 70 mph down a highway.
There's just one problem: the bag covers up my entire back, including the reflective materials on my jacket, which makes me harder to see at night. It would be great if you could run reflective piping around all the black areas on the back of the bag. Alternately, the section just below the top pouch that forms a "Y" shape or the zipper flap over the top pouch would be a great place for a big reflective patch.
I know putting a big silver area on these beautiful bags will make them a little less pretty, but given the choice between pretty and visible at night, I'll take the latter. I don't think it should be a standard feature (though the piping would be pretty unobtrusive), but an option for this would be great.
As it is, I've been working on an alternate way of carrying the bag on the bike. I bought some 1" web strapping and quick-release buckles. I'm going to run the strapping under the rear seat, bolting them directly to the frame, and then connecting the buckles to the top compression strap female buckles. Presto! Instant tail bag that's easy to carry around with you when you go shopping, to school, etc.
There's just one other little problem with the Brain Bag on a motorcycle... its so easy to overload yourself. The bag is so awesome and voluminous that you just want to carry everything around with you.