I just switched our four GSM lines to the National Plan.  We live in the
Northwest and the "local" plan on the GSM is pretty sparce.  By the looks of
the map if we get a few miles off I-5, we would be roaming (for $.69 a
minute).  The plan states that no matter where you are in the United States,
Peurto Rico or the Virgin Islands, you will not be charged roaming.
However, on the Q&A section of the AT&T Wireless website the following is a
little ambiguous:

     Q:  How do I know whether a call will be counted as part of my
unlimited Mobile-to-Mobile calls?
     A:  With GSM America National, your unlimited Mobile-to-Mobile minutes
are available for calls placed to or received from the applicable
Mobile-to-Mobile Service Area to or from another AT&T Wireless customer when
both are on the portion of our domestic network that we own and operate and
"AT&T" or "AT&T Wireless" is displayed on the phone.

It sounds like you won't be charged roaming anywhere in the United States,
BUT it sounds like your anytime minutes might be used even when you are
calling wireless to wireless if  you or the person you are calling happens
to be in a "roaming" area.  As for coverage, I haven't traveled out of the
area yet, to see if I hit any dead spots.