If you're an avid traveller, you might want to think about picking up a soon-to-go-on-sale Doodad pay-as-you-go SIM card: it offers flat-rate 3G data around the world and will work in any unlocked GSM device. Australia is included as one of the launch countries too.
Flat-rate doesn't mean you get charged the same in every country, but you can at least set caps for yourself so that you always end up spending the same amount. You'll be stumping up 30c/MB in Australia, the USA and South Korea for instance, while prices jump to 60c/MB in China, Canada and the UK. In the most obscure countries — think British Virgin Islands, Haiti, Samoa — it costs $1.20/MB.
You also get a full year to use credit added to the account, so if you don't use it all on one trip you can roll it over to the next. Doodad is currently looking for volunteers to help launch the service in 54 counties around the world. If you're travelling in March, go fill in the form — you could get $US300 worth of data to use during your travels as you see fit. [Doodad]