Keith wrote:

We will be road travelling through the western US next month. I have a
tri-band mobile phone, but think that global roaming charges (from
Australian account) may too expensive for regular use. Are there fairly
cheap pre-paid phone or SIM card packages avaliable in the US, and if so
where are they available and what might be the approximate costs? What about
hiring a phone from the rental car company?

Assuming your mobile phone uses GSM-Technology: might
be worth first to check gsm-coverage for the area you plan
to travel at www.gsm.org more specifically
http://www.gsmworld.com/roaming/gsminfo/cou_us.shtml
otherwise you will find yourself with a gsm prepaid package
and still pay roaming for having to use an other service
providers network. Or at least you can check roaming
partners to your service provider in the US and the prices.

I have no first hand experience for the US, but have given
up such plans for trips to Canada (average once a year) as
prepaid SIM-cards are offered with very limited services at
relatively high prices (compared to Europe) - for example,
international calls limited to a very few countries only .... .

Looks like its less expensive using international roaming
on a decent contract with a national provider - or, what I try
to do on such travel, use SMS whenever possible.

If you expect to do lots of "local" calls, renting a cellular
phone might then be the appropriate solution (not necessarily
gsm!)