Here is a few roaming service offered:

http://www.nextel.com/en/services/ca...ming_pcs.shtml
http://www.wireless.att.com/learn/en...l-roam-pop.jsp

Due to the time zone difference, you may want to turn off the phone while you sleep.

I used PDC and GSM phones before and carried three phones as I traveled. Since Japan started UMTS service, now I carry 2, my Sprint CDMA and a UMTS for everywhere else.