I've bought a number of lens from the US including my 70-200 2.8L IS.
Even having it go through customs and paying the 10% GST and handling/insurance costs (full description and cost i.e. no GIFT label), it is still WAY cheaper than buying it locally (great now with AUD = 0.90USD, maybe time for another buy....)
Surly Canon Australia (CA) and Canon USA (CUSA) pay about the same for their products from Canon HQ and shipping costs to Australia would be about the same, if not a little less than shipping to USA.
I think that massive price difference between the two countries is that CUSA make profits by moving a large volume of products while CA make their profit by higher margins as they don't sell near the amount CUSA, hence the much higher prices here.
Just my 2c worth...