[I tried to post this before and don't think it went through, so here it is again. ]
I live in the USA and can give more info on pre-paid options here. The poster who said US pre-paids don't work in Canada is right, at least for AT&T phones (mine doesn't). Also, pre-paids don't have quite the coverage area that postpaid phones do. They appear to be on slightly different, less comprehensive, networks (I don't know why). In very remote areas (away from highways and towns), it's possible that your Aussie phone on roaming will be able to make a call, when your US pre-paid won't, so I'd advise holding on to both SIMs while travelling just in case.
Many US pre-paid phones are now GSM and do use SIMS (AT&T, T-Mobile and some TracPhone). However I've never seen a SIM for sale alone -- just with a phone. You'll typically pay something like $24.99 USD for a phone and an AT&T SIM, generally with $10 or more USD airtime credit.
Sprint, Verizon and Virgin Mobile in the US (which uses Sprint's network) don't use SIMs.
Pre-paid phones are available in electronics stores like Circuity City and Best Buy (super-sized Harvey Norman / Crazy John's equivalents), discount stores like Target, or some grocery stores. Be warned that you'll need a machete to liberate an AT&T go-phone from its plastic container; I've seen TracPhones packaged in a more friendly fashion.
I like AT&T's "go phone" pay as you go service, which allows for refills via card (bought in same places as phones) or the Internet. As others have noted, it's $1/day that you use the phone, plus 10 cents a minute for incoming and outgoing calls. Of course, you'll have to couple it with a calling card to call back to Australia.
Some extras that may make the AT&T worth it are:
1. For $20 USD you can get 30 days of unlimited night and weekend calls, incoming and outgoing. Given that US evenings are a great time to find Aussies awake, this can be quite a deal if you think you'll spend more than 200 minutes during your trip phoning home in the US evening/night. All you pay on top of that $20 is $1 per day that you use the phone and your calling card fees, which can be as low as a handful of pennies a minute, for outgoing calls. Incoming calls during the US evenings are free with this $20 plan.
2. For $20 USD you can get 30 days of Internet use on your phone. If you have a smartphone and plan to just move the AT&T SIM into your usual phone, this can be handy. You can read up on these two options by searching for them on the slickdeals dot net website, but note that you can't just purchase them online -- you have to call AT&T customer service and ask for them.
As far as for people calling you from Australia, as a repeat visitor to Australia, I recommend the OzCall card, which is just a few cents a minute to call the US. (It was originally recommended to me by a coworker whose son is in uni in Australia.)
I've been an AT&T pre-paid go phone subscriber for a year and given that my cell phone use is usually in the evenings, it's the best deal for me.