It's interesting that across different banks (and even across different merchant agreements within the same bank) there are differing approaches to 'premium cards'.
My merchant facilities are with Westpac, and we pay a flat rate for all Visa or Mastercard payments, regardless of the colour of the card. At our last rate negotiation my bank manager did mention that the practice of differential surcharges was becoming more common, predominately due to the increasing capabilities of the EFTPOS machines, but I would rather a flat rate across all cards than a differential rate which varies wildly depending on which card is wielded by the customer.
As a merchant I've been semi-seriously toying with the idea of implementing a surcharge for cash and cheque payments. I hate the outdated 'deposit slip in triplicate' process of banking cheques and hate the risk of staff carrying around thousands of dollars of cash which I then need to cart down to the bank.
My understanding is that Westpac does not have the capability at this point in time to charge different rates for different cards, other than corporate cards. This may be an EFTPoS terminal constraint, or a system constraint.