I was excited when I got my iPhone 5 to read that the Verizon version was unlocked for overseas use, though this isn't a fact that Verizon advertises or will even tell you about. It was noted on tech blogs. I was curious to see how this would work in practice.
So in case this info helps anyone, here is my experience when traveling to South Africa recently:
I took the iPhone 5, purchased last month, and a paperclip for removing the SIM card. I didn't do anything special to notify Verizon that I was traveling and I didn't activate their global plan. Instead, when I got to Cape Town, South Africa, I asked around for a cell phone store that would sell me a local prepaid SIM card. It had to be a nano chip since that is the new size the iPhone 5 uses. It took an hour or so, but I found a Vodaphone reseller that sold me the chip, with 250MB of data and a limited voice allotment. They took care of setting it up, basically just removing the Verizon SIM card and putting in theirs. I carried a small Ziploc bag to keep the Verizon SIM safe for replacing at the end of the trip. (Because the Verizon iPhone 5 uses the CDMA technology inside the United States, I am not sure the SIM card is necessary for use at home, but I didn't want to lose it anyway.)
The phone worked great! It clearly was unlocked or it would not have activated for service from a different company. I paid the equivalent of about $25 for the week's worth of service. It would have lasted up to a month but I wasn't staying that long. It was very convenient in keeping up with co-travelers to be able to use the maps, send texts and emails, and place and receive phone calls. This was cheaper than using the Verizon global plan, though if I had been traveling around to many different countries it may have been preferable to go with their plan rather than getting a new SIM for each country. Also, some places may not have nano SIM sizes available, particularly since the iPhone 5 has not been released everywhere yet. Cape Town may be a best-case example because they have so many international travelers.
To be clear, everything I have read says that the Verizon iPhone is only unlocked for use out of the United States.