Sorry if this is covered elsewhere, but iPhone 5 Verizon working fine on AT&T
It appears that what Verizon said about the "5" being an International phone was correct. I just cut down my AT&T sim card (still active, but old account) to nano-sim size, popped it into the SIM slot on the Verizon iPhone 5, and up came my AT&T number and all capabiities, including "4G" data. Easy Peasy. I'll try a bit over the coming days to see if the AT&T side is capable of HSPA+, and to what speed, but I need to drive around a bit to get into areas that i know provide 8Mbps, 21, and 42 MBps over HSPA+. I do know that this SIM (from my Samsung Galaxy Nexus) did do HSPA+ 21 but we'll see.
Since that worked, I also popped in my SIM card from Panama (after reducing it to nano-sim size), and it also "worked" though no connection, etc. due to distance, but it did recognize my carrier (Movistar), etc.
Only conclusion possible. The iPhone 5 as sold by Verizon is a true international phone and has both CDMA and GSM capabiities. I can't figure out as yet if the GSM side is quad-band or penta-band, but for us international travelers, this thing is great! Use Verizon's amazing LTE network here, but when you're in fringe areas or in other countries using GSM, just stop at an airport kiosk and get a local SIM card. Don't forget to take your little SIM cutter with you though, and it's various adapters (about 6 bucks from eBay), and you're good to go.
This isn't just evolutionary, it's revolutionary in terms of provider access! Hat's off to the big V for allowing this to happen! (And in case you didn't know, the DID have a choice! ), and they opted for the customer. Wow!
For those that want to try this, the original Verizon SIMS are a teeny-tiny bit thinner than "normal" SIMS. Easy solution is to just plop the cut SIM upside down (gold stuff up) on a "emory board" (ask a woman), use the fine side, wet your finger for traction on the SIM and just "sand" it down a bit on the emory board, few seconds. Once the letters disappears on the sanded side, you're good to go. Still a bit to tight? Just sand a wee bit more, be patient, it WILL work, just don't overdo it. I'll also try this with a T-Mobile card, I've got an active one around here somewhere, though I'd not expect the results to be very different. Will report if they are.