I assume that you are from Australia.
If your Australian cell phone is an unlocked "tri-band" or "quad-band" phone (that is, with GSM 900/1800/1900 or GSM 850/900/1800/1900) then you can use your Australian cell phone in the USA and Europe. All you need to do is buy a prepaid SIM card in from a cell phone provider in the country that you are visiting. For the USA, the big two GSM providers are T-Mobile and GSM.
If your Australian cell phone is unlocked, but just dual-band (GSM
900/1800) then it is useless in the USA but you can use it in Europe. You will have to buy a phone in the USA.If your Australian cell phone is locked, see if you can get your Australian carrier to unlock it for you.
Of course, if you do not mind paying exhorbitant roaming charges, you can use your Australian SIM card in the USA and Europe. Presumably, you're asking about pre-paid service so you don't have to pay roaming. Ask your Australian carrier for more details. Foreign roaming rates start at about US $1.25/minute, but if you only make one or two calls that would be cheaper than buying a pre-paid SIM.
If you find that you must buy a prepaid phone in the USA, there are several carriers which offer prepaid service and cheap prepaid phones. You can buy prepaid phones at many places, including grocery and convenience stores, but activating the service may be difficult.
As elsewhere in the world, the social problems caused by anonymous prepaid phones have caused many USA carriers to require some form of identification for their prepaid customers. Basically, they want some means of identifying the user of the phone in case the phone becomes involved in illegal activities. This is not just the USA; Japan, Canada, and the UK are also adding identification requirements.
So, your best bet is to go to a company-owned cell phone shops, explain that you're a foreign tourist (have your passport handy) and ask if they will sell you a phone. Pay with a credit card; that serves as excellent identification. Alternatively, if you have a friend in the USA, ask that friend to buy the phone for you and register it in his name.
Assuming that you're buying a phone in the USA, I would recommend against the GSM carriers. GSM is primarily an urban service in the USA, and coverage can be spotty or non-existant outside of the large cities. On the other hand, it sounds like that's more or less where you'll be the entire time.
I recommend Verizon as having the best overall coverage; they are the largest CDMA network. Try to buy a phone with dual CDMA/analog coverage. Although analog is rapidly becoming less important than it once was, there are still analog-only parts of the USA.
You'll find that the topic of which carrier is the best is rather hotly debated. Verizon is more expensive than the others, and has a somewhat stodgy reputation. Verizon certainly has data acces, but you may have to buy a somewhat more expensive phone.
AFAIK, Virgin resells SPRINT (Verizon's primary CDMA competitor) service and has good pricing on both phones and service. I don't care for SPRINT as a company, but that's my own prejudice.
There are also some companies (e.g. Tracfone) that sell cheap analog-only phones or TDMA phones. For a short-term visit this is a good deal, but both analog and TDMA are on their way out. Forget WAP or any other data access with these.
Another reason for going with Verizon is you buy a phone in the USA is that a US GSM 1900 phone is of no use outside of the USA and Canada. A US CDMA phone, on the other hand, *is* useful in a few other countries besides the US, including Korea and China. Verizon does
*not* lock their phones (the security code is 000000) so the phone can be reprogrammed and used in other countries. The other carriers lock their phones.I would recommend against rental. Renting is almost always much more expensive then buying a throwaway prepay phone, and the pre-minute rate isn't much less than roaming.
By the way, if you ever find yourself visiting Japan, I recommend going with Vodafone. They're the most gaijin-friendly of the Japanese cell phone companies, being a foreign-owned company.
Correction to previous message: the two big GSM carriers in the US are T-Mobile and Cingular.
-- Mark --