Cost of the bandwidth is one issue. The amount of time it takes to download is another. I have a 4mbps connection but power cuts and fluctuations makes it difficult to do a 4gb download from Apple. Every upgrade of Xcode is a horror. The download stalls in the middle and then fails to re-continue because the session timed out. I have to end up doing some gymnastics by exporting cookies from the browser to a file and then coaxing wget to use that.
I hope they've got a decent pause/resume system on the Mac App Store downloads.
Speaking from Australia, I'll assume that at least iinet will have it on their cap-less servers to download. For example most major Steam games are and don't count against the cap.
And lets not forget linux mirrors which iinet also provide unmetered. I'm not sure if they will be able to mirror an update to Lion though, that would require some talks with Apple.
