I've had the Verizon iPhone 4S since it launched last Fall, and I've now taken trips to three countries to use it. I've spent extensive time in Belgium and New Zealand using local micro SIM cards to great effect, and due to a Verizon snafu I've had to use their international service in Belgium as well and I can report it indeed is grossly overpriced. I figure I would share a couple tips and do a quick review after a few weeks abroad.
Step 1: Call Verizon after 60 days and ask for an "International Unlock"
The key step before you go abroad is to make sure you own your Verizon 4S for at least two months and then call customer service to request an International Unlock (I found out about that at ZDnet). They will try to pitch you their international calling plan but refuse it or say you will consider it if the unlock doesn't pan out.
You have to do this over the phone, the reps I talked to inside a Verizon store couldn't do the procedure. Also, make sure the person on the phone knows how to do this, because the first time I called it was the person's first attempt at ever doing it and it turned out they didn't complete the task properly, causing problems later on.
Step 2: When you get off the plane and through customs, look around the airport for SIM card kiosks
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.
Step 3: Carry a paperclip and a holder to keep your old Verizon SIM in when you're not using it
I have an old SD card case (pictured above) I carry in my travel backpack with all my old micro SIMs and a paperclip. It's really easy to pop the paperclip into the side of your iPhone, slide out the tray, and drop another card in. Every card has its own unlock code and you have to be sure to remember them because they can get locked out from use if you fail on four attempts. It's also fun to feel like a character in The Wire as you can jump from SIM to cheap SIM, switching to a new number each time. Be sure to keep your original Verizon SIM for when you get back home to the US, otherwise you'll have problems.
Warning: don't use Verizon for international use
Since my first attempt at unlocking didn't work, my first use of an International SIM in Belgium didn't quite work out. I could get onto the new network, but I couldn't make calls or get any data. I eventually had to pop in my Verizon card and call Verizion to have them double check (and I also enabled an international plan in case that didn't work out). Data cost me $20.54/Mb in Belgium as I checked the Verizon.com site for a contact number, then I had to wait on hold at several dollars per minute. When they fixed my unlock, new SIMs worked fine, and in the meantime I tried out another local provider and it seemed to work too. I also had no trouble using my Verizon phone in roaming mode in Canada while at the airport, using data from Verizon's own plan.
Verizon's top international plan costs $125 for only 300Mb of data (On AT&T last summer, I used their $200 800Mb plan for two weeks in Australia without going over) and thankfully Verizon counted my international call/data time before I enabled it into my $125 option (I didn't have to pay the $20.54/Mb price).
Bonus: crazy local numbers aren't all bad
You might want to keep your phone's local US number when abroad (especially when traveling with other Americans that want to call you easily), but I'm more of a texting person and thanks to Apple's iMessage feature, I could text any other American I knew while traveling using data instead of my SMS allocation. Getting a local number proved handy for having a way for local people to contact me without them having to use international talk time as well.
Beware of international data hogs
One last tip: in my experience and talking with friends, it seems like Google Maps is the worst culprit when it comes to data use. It is super useful for getting around, but all those map tiles quickly add up. I was lost in Sydney my first afternoon in Australia last summer and by the end of my first 24hrs, I had amazingly used up 200Mb of my 800Mb allocated for the 2 weeks. After that I took the advice of friends and moved to using OffMaps as much as I could, which uses OpenStreetMaps along with your GPS location to give you a good idea of where you are without tons of network use (you download your maps on wifi, then use them in an offline way).
It's also a good idea to take advantage of free/cheap WiFi in cafes, hotels, and at business offices as much as possible.
Conclusion: international unlock rocks
Considering that in Belgium I got more than Verizon's top plan for the equivalent of $13, it's a no brainer: get your Verizon 4S unlocked and always go with the cheap local SIM option. It's quick and easy to get a local SIM at the airport, and pop it in. The first time I did this, I had to be on WiFi and connected to iTunes to "activate" my SIM slot, but my last trip to New Zealand didn't require that and a new SIM worked fine after popping it in.