CHOOSING A CELLPHONE PROVIDERThis document is meant only as a general guideline for choosing a cell-phone provider and a cellular phone.  Since this is a rapidly-evolving area, this info may not be up-to-date.
Recently, many affordable nation-wide no-roaming fee/no-toll plans have become available.  Some of them are as inexpensive as $25/month which includes 75 minutes of use per month with no roaming and no long distance charges.  With prices like this, cell phones are now within economic reach of most of us.
When choosing a cell phone provider, make absolutely certain that you will NOT incur any roaming or long distance charges ANYWHERE in the US on ANY SYSTEM.  The last time I looked, some providers - such as US Sprint - had free long distance and no roaming fees ONLY WHEN ON THEIR SYSTEM.  Avoid these wireless providers!
Some of the major players are ATT, Verizon and Cellular One.  Almost all providers will require that you "live" or have a billing address in their local coverage area.  This is not a problem if your mailing address is within their coverage area of if you have a relative's address that you can use.  You will probably want to select a provider that has a number that will be a local number to call for you relatives so they can call you toll free.  Note that you can purchase your cellular service and phone with a Portland phone number even though you are physically in Miami -- assuming that your chosen cellular provider operates in both Portland and Miami.
Most cellular phones require a special cellular-capable modem, although some newer models do not.
In the (hopefully near) future, we will have much faster, affordable mobile internet access via satellite that doesn't require a land line for the uplink and CDPD.
There are several things to be aware of as you use your cell phone:
First, using your cell phone in Canada or Mexico can be VERY expensive as you will rack up roaming and international charges.  You may want to seriously consider using your cell phone outside the US only for emergencies.  If you plan on using your cell phone in Hawaii or Alaska, verify with your wireless provider that you will not be billed for roaming or long distance when in these states.
Second, your roaming usage minutes may not be reported to your wireless provider in the BILLING MONTH they were incurred.  Therefor, you can easily go over your monthly minutes on the next month and incur still per-minute charges.  Some wireless providers let you carry over unused minutes to the following month, usually for a fee.  However, most wireless providers do not let you carry over unused minutes and further state in the contract that airtime minutes may be billed in the next billing cycle when roaming, which means that you could be responsible for per-minute charges.
Some users of ATT Digital One Rate report that their service has been cancelled because too many of their calls were made outside of their home area.  Some report that ATT has demanded two documents proving that you actually live in their service area.  Be up-front with your wireless provider.  You don't want any surprises later, especially since changing providers may also require that you purchase a new phone.