If you have a multi-band "world phone," ask Cingular to unlock your phone (for
international travel), and then you'll be able to use prepaid local SIMs in
the different countries you visit.  Otherwise buy an unlocked "world phone"
(e.g., on eBay), and use it when you travel.

For data, you'll need to be sure that both the phone and the prepaid local SIM
account support CSD and/or GPRS.

Bon voyage!  I'm jealous!

One problem for you with GSM is the limited real coverage of
individual cells.  Because of timing constraints, the usable
range is limited to 35 km from the base station.  You may get
strong signal much further away than this (especially at sea),
but the phone will not work (neither for voice nor SMS).

There are a very, very small number of extended range GSM cells
world-wide, which will work to 72 km.  These do not support GPRS
data.  I mention them only to prevent someone correcting me.

GSM World (http://www.gsmworld.com/roaming/index.shtml) has
information on roaming, as well as reasonably up-to-date coverage
maps.

You'll need a phone with enough bands to cover the international
frequency range.  That's at least tri-band (900, 1800 & 1900
MHz).

Just buy the pre-paid pack when you arrive (usually widely
available, even from convenience grocery stores).  There'll be a
holder for it in the back of the phone, probably under the
battery.

http://www.mobileshop.org/ is another very informative (UK based)
GSM site.

John

Your best option is to use Cingular, but be sure to get an unlocked
quad-band phone. Cingular has good roaming rates in Mexico. Not sure about
data access via cell phone down there, I kind of doubt if it would work.
Note that Cingular has recently been refusing to unlock world-band handsets,
so you may have to buy an unlocked quad band handset on-line (high prices).
Or you can just buy a separate unlocked handset (900/1800 Mhz) for Europe
and Asia, these are inexpensive.

North America: GSM: 800 Mhz/1900 Mhz (sometimes 800 Mhz is called 850 Mhz,
but it's the same thing)
Rest of World with GSM: 900 Mhz/1800 Mhz.

Off the U.S. coast, AMPS will have the greatest range from shore, so you may
want to get a pre-paid TDMA/AMPS phone from e-Call Plus. These are cheap.
This is only if you'd like access close to the U.S. coast, but you'll haver
radios for emergency use I presume.

In the Bay Area, GSM service is pretty bad (consistently ranked the worst in
coverage, other than Nextel), but if you want a single handset that will
also work in Europe, then you're stuck with GSM (many people, including
myself, use Verizon CDMA in the U.S., and keep an unlocked GSM handset for
use in Europe and Asia). As my old boss said, "on Cingular, I can call my
wife from Beijing, but not from Santa Clara," (of course in Beijing he was
roaming onto another carrier).

In Europe and Asia, it's much more economical to buy pre-paid SIM cards and
stick them into your world-band phone, then to roam internationally. As to
where to buy them, check out http://www.prepaidgsm.net/ , more specifically,
http://www.prepaidgsm.net/en/operators.html. It'll tell you more than you
want to know! In Taiwan, I buy them at convenience stores such as 7-11. But
remember, you'll have a local phone number, and you won't be roaming and
receiving calls to your U.S. number. Be certain not to turn your phone on
with your U.S. SIM card installed because the U.S. carrier will start
charging you international roaming, even for calls to your voice-mail.
Better to leave a message on your home phone machine with your international
number.

Data services mau be unavailable with pre-paid SIM cards. Bring an 802.11
equipped PDA (i.e. Toshiba, Dell), and get a list of free 802.11 hot spots.
Too bad the new generation of UPCs won't be ready by the time you go (Ultra
Personal Computer).

There are also call-back GSM services, that work with specific handsets.
This is a good option if you'll be in a lot of different countries, because
other pre-paid SIM cards, while cheaper, are country specific. The way the
call-back system works is that when you call someone, you're actually
calling the service provider and sending them (automatically) the number you
wish to call. Then they call that number, they call you back, and connect
you together. This takes advantage of the fact that it many, if not most,
other countries, incoming calls to cell phones are free. The disadvantage is
that your phone number isn't local, it's in Luxumborg (sp?). And again, it
only works with handsest that are capable of being programmed to divert any
call to the service provider and transmit the number you are calling (they
have a list of compatible handsets on their web site).

See: http://www.earthroam.com (one of my sites) for a lot of information on
worldwide roaming.

Steve