I can confirm that the Verizon iPhone 5 is GSM unlocked and works as you would expect overseas, at least on the networks I visited during a recent trip to England, France and Belgium. I had to purchase a prepaid micro-SIM and have it cut down to nano-SIM size to fit into the iPhone 5 but the process was quick and painless.
In London, I visited 3 mobile, T-mobile, Orange, O2 and Vodafone, and none of them were offering prepaid nano-SIMs when I was there. In order to get the nano-SIM you had to sign a contract and use a UK credit card and billing address.
I purchased a prepaid micro-SIM from 3 mobile for £15 and took it to one of those small mobile phone shops that sells phones and plans from many different carriers and advertises phone unlocking services on a dry-erase board out front. They are plentiful in London and both places I visited would cut the micro-SIM to a nano-SIM using a little purpose-built cutter. One vendor wanted £10 to cut the card. The other wanted £3. Ask around, it seems negotiable.
Once the card was cut, I popped it into the iPhone 5 and squirreled my Verizon card away for safe keeping. In a few seconds, the 3G indicator came on and I was surfing.
I was able to use the same SIM card in France and Belgium but I elected to turn off data for that portion of the trip to save a few bucks since I would be roaming and that requires extra fees. The networks in those countries were recognized and worked though. I would have repeated the card cutting process if I had planned to stay longer in those two places.
If you're planning to do some country hopping in the very near future, you way want to invest in a nano SIM cutter since you'll probably want to get a prepaid card for each country you visit. Hopefully the elusive prepaid nano-SIM card will become plentiful in a few months.