I've just finished my latest trip down to Mexico City.
The bad news: DCC has continued to spread like a very bad weed. Almost every restaurant I went to in the Polanco neighbourhood had DCC in their terminals now. From the many trips I've had to make down there over the last couple of years, I've been watching DCC grow from nothing to almost total presence in the areas of Mexico City that are popular with foreigners.
The good news: on every single occasion the servers were more than happy to cancel the option and complete the transaction in Mexican Pesos. No fight, no confusion, no problems. So even though DCC is becoming ubiquitous, the front-line staff seem quite happy to bill in Pesos and don't seem to want to trick customers into DCC.