I work in Louisiana about 40 miles from the Gulf of Mexico and have been hit with roaming charges up $2000 from Cingular. When I first saw "Cingular Extend" on my phone I called them and asked if I was roaming and the rep. said, "No, you have nationwide and you are in the USA." Then I got a $2000 bill. After much discussion they removed the charges but told me not to make any calls again if my phone said "Cingular Extend". I followed that rule faithfully but was charged roaming again and when I inquired they told me my phone had no way of knowing if it was roaming.
Here are copies of two emails from Cingular about this problem.
Remember Lousiana is many miles from Mexico or Canada.

I understand you received international roaming charges, but you stated
you were not in Gulf of Mexico. Your phones display does not indicate
the rate at which you will be charged. Charges are based on the location
of the site receiving and transmitting the call, not the location of the
subscriber. Fringe service areas are those areas where coverage by two
cellular operating carriers overlap, examples of such may be found in
areas near Mexico, Canada, or the islands in the Caribbean. Overlapping
coverage is common because there are no distinct service lines between
competing carriers' cell sites, and may result in roaming charges even
though you have not traveled outside of your coverage area.
Unfortunately, the international roaming charges are valid. We do have
packages available for those who travel frequently that will allow you
to enjoy discounted rates. Please visit www.cingular.com/cingularworld
for more information.

We at Cingular Wireless appreciate your business and value you as a
customer.

Later.....

Your rate plan is the Nation plan and you will not be billed roaming in
the United States. Your phone may not display extended because you in
your calling area. Some International towers put out a roaming signal
and some do not. Cingular does not have access to those towers to make
them send out a roaming signal. There is not way to 100% verify if you
are in that same area, if you pick up an International carrier unless by
experience you know how far in you can go. If you need further
assistance or clarification, please call customer care at 1-800-331-0500
or you may reply back to this email.

So basically what they are saying is that if you have the nationwide plan you still have to pay for roaming and there is no way of knowing if you are roaming until you get your bill. As the representative said you have to go by "experience" and believe me this education is very expensive. Also, it appears that the state of Louisiana is near some international border. Which foreign country would that be? Texas? Mississippi?
Greg