Matt Haughey has this fantastic entry on how to travel the world using your own iPhone — when unlocked — and to take advantage of the cheap telephone service from abroad. If there is one thing he has learnt is that mobile services in North America are insanely expensive and don't give you nearly as much as you would get abroad.
In Belgium after I got my bags and before I left the airport, there was just one little booth selling cards from BASE.be, a local mobile provider. They were fantastic, offering 500Mb of data plus 15 Euro of talk time and texting for only 15 Euro. I also set up topping up the card via SMS, so after a few phone calls and some heavy downloading on my laptop (you can tether the connection) I could simply send a text to get more credit.
In New Zealand's Auckland international airport, there were two options, Vodafone or Telecom and I selected Vodafone since I'd heard of them before (seen them on soccer jerseys). They offered 250Mb of data plus a bunch of texting and calls for $45NZ. I used up all the bandwidth halfway through my week and topped it up again via SMS.
My local Verizon rep said the USA is the only country in the world that requires you to have an expensive phone plan with hundreds to thousands of minutes of talk time plus data plus texting. Every other country does fine with these cheap pay-as-you-go cards. I would LOVE to have the same setup I had in Belgium, where 10 Euro could last me weeks of heavy phone use instead of the $70/month plan I have with Verizon.