In Brazil, Diners Club and MasterCard have been accepted at all of the same merchants for as long as I can remember. Neither card bears the other's logo, but they both use exactly the same point-of-purchase terminals. A few VISA terminals used to accept American Express, but this is no longer the case. Nearly every big merchant has three terminals: one for DC/MC, one for VISA, and one for AMEX. In Chile, Diners, Master, and VISA all seem equivalent, with AMEX separate from the rest.
I'd have to think about other countries, but the union of Diners Club with Master Card is not a new idea!