The GSM networks in the states cover only major traffic routes and larger population centers. Hence, if you're using a GSM (e.g. T-Mobile) phone in the US, you have no service in most rural areas. No service = no calls = no roaming.
The CDMA networks (Verizon and Alltel being the two largest) are structured to cover greater land area and typically if you want rural service your only option is CDMA because of the two different network design strategies. Generally speaking, in the Seattle area the coverage I have on my CDMA phone is vastly better than the GSM coverage my friends have. Still, in most urban centers you won't notice a difference, and unless you frequently find yourself in places far outside the city, you don't need to worry.
ZV