You CAN'T get both miles and the DC 20% discount. And dgordon never said that: "I have found some very good restaurants through the iDine program, including the 20 mile restaurants. Some of those also accept DC, so those give 40%, and come out cheaper than the cheaper restaurants." What that meant is simply that those restauarnts that WOULD give 20 miles per dollar IF you were using an airline program (in which case you could only pay with Visa, MC, Amex, or Discover) will INSTEAD give you 40% cashback if you pay with Diners Club.

But if you pay with Diners Club, you have no opt in or out, you're always automatically getting the cashback (if the restuarant participates). It's only in the case other cards that you can choose between cashback or any number of airline programs (or even other types of programs like uPromise).

No matter how you work it, any one card can only earn into any one incarnation of iDine at a time. That's because iDine is one system as far as gathering info on where you card was used, and only once they figure out that you used their card do they route those earnings into one program or another. That routing system is smart enough to know that you can't route the same earnings to two places at once, just as your bank is smart enough to know that if you depost $100 in cash you can't deposit the same $100 into several different accounts and have them all grow by $100 each!

There are ways of sort-of double-dipping, but they're not by making the same card earn two ways at once through iDine. One is, of course, that your card may earn mileage on ALL purchases, quite independent of what it earns on iDine. The second is that some restaurants may accept coupons or other kinds of discounts in conjunction with iDine (tho I think they can disallow the combination of coupons or discounts with iDine if they so desire).