[PhotoJim]

I agree with the Hotspot @Home with T-Mobile idea. T-Mobile prepaid is cheap to keep running and it has really good US coverage. If you don't need immediate access to phone calls and SMS in Australia, just timely access, then being able to use it via WiFi is pretty practical.

These new hybrid SIMs with UK and US numbers might work well, too. I don't know what the cost to use them in Australia will be, but you'll apparently be able to get SMSes and voice calls there.

Another option is to try to get a Google Voice number. You can receive SMS by it. You can check your SMSes online, perhaps even by a mobile phone. Voice calls can be forwarded to your Australian number when in Oz, and your US prepaid when in the US.