Wow. Lots of ideas here! This is turning out to be a rather interesting discussion, I must say.

Originally posted by Inverarity:
Not literal tunnels dug through hundreds of miles of earth, of course, but some sort of infrastructure must exist, which would preclude, for example, the Floo Network from working across an ocean (or under the Channel).
Originally posted by Equinox Chick:
Excuse me for being dim, but why does this preclude it working across an ocean, or at least the English Channel which is only 23 miles? The infrastructure is just a network of possible connections, like a wireless system. I think the stargate or teleport system from Star Trek is similar.
Hmm. I don't see the problem with Floo-ing over the Channel - after all, Harry and the Weasleys Flooed to Hogwarts (in Scotland) from the Burrow (anyone know where Ottery St. Catchpole is? I'm going to say the Midlands), which is feasibly considerably more distance than over the Channel. I just wonder whether the water might break up the connection. In any case, I don't think that Flooing from England to, say, Japan is feasible without a number of stops. There's just too much ground - not to mention sea - to cover.

Overseas Apparition, on the other hand, is something I can see working - just as long as you know exactly where you're going (as the opaleye pointed out). One of the "three D's" is Destination, isn't it? Anyway, in the Middle Ages, when Columbus and Marco Polo and all of their chums were gallivanting across the ocean, maybe the idea of Apparition just wasn't very evolved yet. Maybe there wasn't a direct term for it, and wizards didn't know the exact method. Furthermore, even today, splinching yourself whilst Apparating internationally would not be fun, or easy to sort out. (Imagine Apparating to India and finding out that you've left a leg in Bristol!)

Keep the ideas coming, they're really helpful!