Communications in the field is always a problem. GSM is the cellular technology that is used in most of the world. In the USA, Cingular and T-Mobile are the only providers with GSM networks. If you travel a lot, this list of GSM phone frequencies can come in handy:
http://www.celluloco.com/products/customer/pages.php?pageid=18 (died due to link rot) or here: http://global.yesasia.com/help/topic.aspx?topicId=1164&lang=en or http://kbs.cs.tu-berlin.de/~jutta/gsm/gsm-list.html. If link rot kills them all, there's always google (updated 2006.04.17)
I have a T-Mobile Motorola v-330 which is quad-band, so it works pretty much anywhere except Japan. One reason I like T-mobile is that they will unlock your phone after 6-months of usage, which means you can pick up a local SIM card and use that during your travels rather than burning up roaming minutes. And I think T-Mobile has great customer service.
In Japan, the cheapest is to get a pre-paid cellular. However, they're getting much harder to find than in the past -- and you have to show some proof of ID. I'm not sure if they'll take foreign passports as proof.