Speaking of the REI Flash 45, do you have the new Flash 45 by any chance? I tried it on at REI the other day, and I liked it except that it didn't seem to have a real frame. It had an aluminum loop at the top, but the rest along the sides was some type of flexible plastic. For me, a real frame is a must, hence my interest in the Arc Blast.
I was asking about the difference between an Arc Blast with a frame and a Blast with a frame, because I had seen someone post something here about the option of a Blast (not Arc) WITH a frame. I suppose with that setup you'd want some sort of foam against your back for comfort, though.