moore, the barcode they need is the one from the cardboard box not from the camera itself... i have sent mine in and have received my cashback... my barcode was on the underside of the box and i had to cut it out making a hole which doesn't look all that great, but hey, canon gave me £100 for it so what the heck.... seriously though, don't try to peel it off as it will most definitely tear, i have heard of this happening elsewhere and canon contested the validity of the claim...
just a bit of advice, make sure you photocopy everything your sending (i even photographed the box with and without the barcode)