One thing to keep in mind, for those in the US, is that different providers use different technology. Cingular, AT&T & TMobile all use the same - GSM. Sprint and Verizon each use something else. So, if you have say TMobile and are in an area that only has Sprint and or Verizon coverage, you're out of luck. The technologies are not curently compatible. Wether they are really not compatible or the various companies have decided that they wont be compatible, I don't know. Hope this makes sense, I'm fighting a nasty cold and having trouble keeping my thoughts straight.
Leo