For worldwide use you need a GSM phone. In addition (I guess) to Nextel, AT&T and T-Mobile have GSM in the US. In my opinion, AT&T is somewhat misleading about the world capabilities of their phones, and their international roaming rates are higher than T-Mobile's (and include a monthly fee, where T-Mobile does not have a Monthly fee).

Here are T-Mobile's international rates:
Europe*: $0.99/minute
Asia**: $1.49/minute
Malaysia: $1.99/minute

* Includes France, Italy, Ireland, UK, others
** Includes Thailand, Singapore, others

For a complete list go here: http://www.t-mobile.com/international/coverage.asp

First time I've noticed: Japan is not listed as a T-Mobile international roaming country. But for the others T-Mobile will be cheaper (and you'll be able to get a cheaper plan - my plan is $29.99/month for free nationwide roaming, unlimited weekends, 300 anytime minutes) and you might be able to swing a free worldphone as well.

edited to add: Scott's message reminds me that Japan isn't a GSM country - so that's why no GSM roaming there.

I looked at the phones on T-mobile - you can get a dual-band (900-1900 MHz GSM) Samsung R225M for free, but a tri-band phone(900-1800-1900 MHz) will give you slightly better coverage for $99 or $149. I had no problems with a 900-1900 GSM in/around London last month.

[This message has been edited by pdhenry (edited 05-16-2003).]