I've been looking all over for this.
Workmate with iPhone and iPad just came back and said that you simply can not do it without a contract and to get one you need a social security number. Still worth searching - I may have found one but it was $200 for 100mb.
Telstra charges around $40 for 8mb. Best bet would be to use wireless when ever available.
I was thinking if you could get data you could buy a skype number here, divert your mobile and run skype on the iPhone 24x7. Would be the most cost effective if you could get a data card.