[We moved this non-local digression from this thread on the Boston board http://www.chowhound.com/topics/358316 ]
I interviewed the owner of the Montrose (Eddy Tabit) and Paul O'Connell over at Chez Henri about their Cubanos awhile back. Tabit immigrated to the US post-revolution and according to him, while mortadella wasn't common in the Havana sandwich, it was frequently used on other parts of the island. Given that the Cubano was likely an import in the first place, (Cuban chefs were training in Europe prior to the revolution and brought a lot of French and Italian techniques into the cuisine.) the "correct" version is sort of a snipe hunt.
Personally, I like the mortadella, it adds a nice peppery bite to the sandwich. On the other hand, I had a place try to foist corned beef on me once (clearly celebrating the unsung Irish influence on Cuban cuisine) and that was just awful. With the Montrose, I'm sure they're happy to leave the mortadella off if it offends you.