I can confirm that these WILL NOT be accepted over the counter or phone by Carphone warehouse!!!
I went in yesterday and tried my luck, it does actually say the day you bought your phone on it and the can instatly see if this was sent out in error - just to make sure they can call up your customer number and see exactly how much you paid.
An email has been sent round from head office saying that the ones sent out in error are not to be accepted.
However becuase of their obvious and glaring cock-up they are sending out new letters today inlcuding a £20 voucher which can be used by anyone.
(I did a search on this and although there are loads of threads I couldnt see this info mentioned anywhere - Those selling or contemplating buying one of those vouchers should think again)