Chronicles of Narnia: C. S. Lewis adopted a chatty style of narration, so he get away with just openly stating the difference between Narnia and England. Horse and His Boy, Shasta had to wear his best clothes and then Lewis says don't worry, in Narnia, best clothes are comfortable.
I'd note that this kind of thing works for Narnia as well because it's a setting that co-exists with modern (well, more-or-less) Earth. Even with the same narrative style, if it was a pure conworld with no relation to ours, I'm not sure he could have gotten away with it.
And to answer the question directly:
Would a comment like that break your sense of immersion, or does it seem natural to you that the narrator knows what's happened in history between when the story's set and now?
Yes. I'm not going to do the intellectual process of considering how this makes sense with an omniscient narrator - what's going to register is that a story set in Ancient Rome just started talking about football.
edited 28th Jan '12 2:05:52 PM by nrjxll