Rang the reseller. No 20" iMacs have hit Australia yet, and there's absolutely no word from Apple Australia about when they're likely to. Considering that I'm entering into a mortgage in less than a month; that it's been at least two months since I made the order; and since I'm unlikely to get any concessions out of Apple in return for the delay, I've cancelled my order.
There's two and a half grand off the mortgage right there.

If they'd shipped 'em a little sooner, sure, but frankly, I have better things to do with my money than hang around and wait.