FYI - I saw this come across the wire today and thought it would be of interest. If the debit card (which Amex doesn't offer BTW) is linked to a rewards program of some kind, then look out!!

Mortgage Payments By ATM Card
DALLAS (03/17/04) -- BillMatrix Corp. said it has introduced a program to allow consumers to pay their monthly mortgage payments with their ATM or debit cards. The service will allow users to pay over the telephone or the Internet without using a PIN. With the ATM/debit transaction, the BillMatrix system validates that the funds are available in the account for the payment in real-time and transfers the funds to the mortgage servicer.