People who travel know this and either buy an unlocked US GSM phone
(850/1900mHz), or buy a multiband phone that includes the US bands.
If you have a EU only (900/1800) phone, you can often rent a US one
for little money or free from your SP.

There also is a question about 3g Networks, there are two different bands
used in different places. I think the US uses 1900mHz, the EU etc use 2100
mHz.

As for coverage, there are in the US two GSM bands. 1900mHz bands cover
about 80% of the population, but around 20% of the area. 850mHz covers
the old AT&T Wirless network coverage area, which is more than 99% of
the US.

However there is no such thing as 100% coverage anywhere with cell phones,
if you want that you have to use a satellite phone. The best are combined
satellite and quad band GSM with roaming agreements, so when you are
within
the coverage area of any GSM network you use that, but if you are in one
of the few (in the US), or many outside, areas without GSM coverage, you
use the satellite.


No GSM phones here... can't remember what the protocol they use here is, but
it doesn't work anywhere else, and nobody else's phones work here. Not sure
what the law here is regarding satellite phones.