In Canada, the "r" in "arse" is more clearly pronounced the further east you go, particularly from the Ottawa Valley onward, with the exception of Montreal. On the islands of Cape Breton and Newfoundland, there is no quicker way to mark yourself as a "come from away" than to say "ass"--unless you're speaking of the beast of burden. In the rest of Canada, it's not a case of rhotic accents, it's an import of the U.S. pronunciation. I wonder if, in the case of the U.S. pronunciation, "ass" is a non-rhotic import from overseas service in two world wars; it is close to the Southern received pronunciation of "aahce."