More than once, I've been given a hand pay and decided to leave forgetting about credit still on the machine. I think I've always gotten it back... once I was even paged and the slot attendant had cashed it out for me! Anyway, point is that you can still walk away from money, even without a credit (or debit) card in the machine.

I agree that dedicated "debit cards" loaded with a fixed amount would be a likely next step on the way to full credit on the machines. For those with casino credit lines, I see no reason why they couldn't give players direct access to credit (via marker) today. A number of the casinos (Caesars, Venetian, Red Rock for example) will today sell you a voucher (ticket) to use in the high limit machines, typically in $1000 increments.