Wireless carriers in the US are in fact required to provide coverage
to a certain percentage of the geographic area their license for a
given area covers (if an 850 MHz carrier) or a certain percentage of
the population their license for a given area covers (if a 1900 MHz
carrier.)

That said, there are some areas that 850 MHz carriers never built out
that either have coverage from no or only one originally licensed
carrier (e.g. the Copper Basin of TN, which to this day has coverage
only from VZW since USCC never built their license out there), or were
picked up by a different carrier after the original licensee let an
area sit unserved for five years (after that time, an area becomes
"unserved area" that other carriers can apply to serve.  One rather
unique carrier, Commnet Wireless, serves areas other carriers never
built out almost exclusively, and only provides wholesale roaming
coverage to customers of other carriers and doesn't sell "local"
service.  It's hard to sell local service when you have cell sites in,
say, Tennessee on a switch in Yuma, AZ! ;) )

There is *no* requirement that carriers have roaming agreements with
other carriers; this creates some situations where there is coverage
but customers of a given carrier can't use it.  This is a rather
common problem with Cingular or Dobson customers traveling where the
only coverage is from VZW (Copper Basin again) since Cingular and
Dobson loathe VZW and vice versa.

-SC