...at Sainsbury's.
I'd heard rumours to this effect on a money-anorak website I visit occasionally, and today I plucked up the nerve to try it.
With a £2 Pampers voucher in hand I went to the checkout -- if they'd challenged me I was going to say, "Whoops, silly me, I forgot to put the nappies in the trolley".
But they accepted it with never a squeak. Next stop, some Pampers changemats vouchers that arrived last week...