When my family visit us in the US from the UK, I've given them a Truphone sim card, good PAYG rates for calls and data, and you can add local US number for $8 (a month, but easy to cancel) if an Australian one is your primary.
Just received a Piranha mobile sim, got it within 3 days of ordering from the UK based firm, comes with US and UK numbers, and cheaper than Truphone (as long as on Tmobile). Family gets that this August! (esp. as I'm paying!)
Both of these offer cheap rates to call back to Australia.
Tmobile actually has one of the best networks in the US, in terms of coverage and speed (and my Tmob vibrant worked well in Australia, so I presume some of the frequencies should match.) It does roam for free on ATT where TMob unavailable, though occasionally had problems with that.
You can get a PAYG sim card from TMob, but then you are likely to be left with a lot of unused minutes, and the credit expires in 90 days without more top ups. There is a new $3/day plan, unlimited calls and 200MB/day. Good deal if you are using significant data, more than a few emails. But will be very expensive for international calls outside the US.
Tru and Piranha last forever, as long as you send a text or something once a year, or so...