Ah ok in that case I can now answer your question better.
If you are coming to Australia for a visit then your best bet will be to go with 3G and once in Australia buy a pre-paid sim from one of our carriers. By May most of them should have microsim and at least Vodafone will. The sim will probably cost about AU$10 + whatever you charge it up with. Charge cost will likely be something around AU$30 for 2GB with a 1 month expiry or something like that (just an educated guess at this point).
Doing this is likely to cost a lot less than "roaming" on an AT&T plan from the US.
Public WiFi hotspots are not big in Australia yet (there was talk of one in Sydney CBD but not sure if it ever happened as I don't live there).
Also with 3G, most of our carriers only provide coverage in high population areas such as capital cities, with fallback to the older CDMA system out of coverage (and at a premium). Telstra, our gluttonous monopoly king, has much wider coverage but the pricing on their services is utterly disgusting.
If any fellow Australians would like to dispute my advice, please do, as I do not claim to be an expert on our mobile networks.