The year is 2009, cell phone technology for consumers has been prominent for 10+ years by now. Its hard to imagine that we are still limited to where we can use our mobile phones due to physical or financial restraints. Case in point: Roaming. The concept of roaming is for when you are in an area not serviced by your carrier; at which point, if another carrier's service can be used, you can jump on their network for an incremental fee per minute. Back in 1998 when I got my first "digital" cell phone, I would often get no signal on my drive from Los Angeles to Needles, California; friends with dual mode phone can switch to Analog signal for more coverage. In the beginning, the mobile market had more carriers in different market zones, and they tend to invest more along the coast and major cities throughout the country. As the cell industry morphed into four dominate players and as their technology improves, the coverage map starts to get bigger and more carriers started to overlap in serviceable areas. Today, its safe to say that if you are with one of the four large carriers in the U.S., then you should be able to make and receive domestic calls without incurring roaming charges since most plans offer a free roaming feature where they've worked it out amongst the carriers.
There are some roaming charges that you do have to worry about in today's mobile environment:
When traveling aboard, and if you use a GSM phone (AT&T and TMobile with a sim card) you are likely able to hop on a foreign carrier's network to make or receive calls. Sprint and Verizon are on CDMA network which is intended for the U.S. only; unless you have one of their newer world-phones (featuring a GSM slot for roaming purposes), you may not be able to use your phone in other countries.
Most phone companies offer an elaborate "world plan" which is a fixed fee plan for traveling abroad. Depending on how much you would use your phone, it might work out cheaper to pay for the fees/minute according to your plan. You should always call to confirm the roaming fee by country for voice minutes and SMS/Text.
* SMS/Text: I rely on SMS quite often when traveling abroad, its much cheaper than voice minutes and gets my communication needs done; just make sure you study up on the country code first if you have to text with a foreign friend or family member
* If your smartphone has a domestic data plan, unless you enroll in an international data plan, I would advise turning off the data function of the phone to avoid any accidental roaming fees. Some mobile applications may be running and refreshing in the background and you may not be aware that it is downloading data, it can be very costly upon your return
For more information on international roaming, here are the links for the big four carriers in the U.S.: AT&T, Tmobile, Verizon, Sprint/Nextel
This is something interesting that I just learned from my friend Dan who'll be taking a cruise ship vacation soon. While you are on open sea, international waters, there are no cellular signals since you'd be too far away from any tower. Some cruise ships have been retro-fitted with cellular service by one of the four carriers. Chances are, the carrier is not going to matter since they are contracted to support all domestic phones (GSM and CDMA). You should inquire with your telecom to get a "cruise" rate per minute before use. Based on my research, its roughly $2.50 per minute while you are on a cruise. Most of the carriers have a list of supported cruise lines on their webpage. I am going to guess this will be similar for when they finally put in cell phone access in the NYC subway.
For more information on cruise ship roaming, here are the links for the big four carriers in the U.S.: AT&T, Tmobile, Verizon, Sprint / Nextel