There is a fundamental difference between US cell phones and the rest
of the world. In the rest of the world, the calling party pays for air
time. As a result, cell phones have unique 'area' codes in the rest of
the world, so you know you are calling a cell phone.

In the USA, the owner of the phone pays for air time, there are no
unique 'area codes' in the USA or Canada for cell numbers.

So the if you want air time in the USA, you get to pay for it.

Having said that, both Cingular and T-Mobile offer reasonable prepaid
air time. Keep in mind however that coverage outside the big city in
place like the 4 corners area is pretty limited.

I traveled in January the East Coast from Miami to NewYork and used a
Triband Phone with a SIM which i have purchased here:
http://www.cellion.de/
sorry the page is in written in german but i think your austrian
visitors can read it.

An other solution may be found here:
http://www.globalsim.net
this SIM is working via callback

In some Regions a phone which can used at GSM850 maybe better. In Europe
Triband cellphones will support GSM900/1800/1900 but not GSM850 maybe in
some US Regions phone supporting GSM850 will be a advantage.

Other interesting links:
List of GSM Provider including Coverage Maps
http://www.gsmworld.com/roaming/gsminfo/index.shtml
Global GSM Coverage Maps
http://www.coveragemaps.com/gsmposter.htm