Hi Ash,
I've bought books from Baen (as above) for SF/Fantasy, but for other books I've used Dymocks (tiny range) and Diesel Ebooks (
http://www.diesel-ebooks.com/). None of them do the "Lets be bastards to the rest of the planet" thing, so are quite happy to sell you ebooks. Formats get a bit trickier, but by reflowing the PDF or using the Epub format, or converting the MS Reader formats it's not a big problem.
As mentioned, there are a number of other sites that use your ISP location, not your credit card number to determine your location. This means that if you're lucky enough to have access to machines in the US (via your company etc) then you can still buy them with your Aus credit card.
The two funniest things for Aussie Sony owners are :
1) Sony bookstore won't work. Clever of them to think that the reader wouldn't get outside the US in this day and age.
2)
www.ebooks.com is a huge ebook web site - based in Perth (5 minutes drive from where I am now) and I can't buy most of their books.
Some days the irony just gets to me.