Can I use the Sprint / Verizon iPhone 5 (A1429) on AT&T's GSM network?
Yes. The Verizon iPhone 5 has been tested with an AT&T nano-SIM card and the device is unlocked. This means you can actually use your Verizon / Sprint iPhone 5 on the AT&T network at HSPA+ (4G) speeds. The device will not operate on AT&T's LTE network due to differences between LTE frequencies on Verizon's and AT&T's networks.
This is also good news for travelers, who can grab a pre-paid nano-SIM card off the shelf to use their Verizon iPhone 5 on any compatible GSM carrier around the world.