iDownloadBlog has confirmed that the Verizon iPhone 5 being sold on contract does arrive unlocked for use on GSM networks, allowing users with a nano-SIM or a trimmed micro-SIM from a GSM carrier such as AT&T to use the device on their networks.
I can confirm that the Verizon iPhone 5 is indeed GSM unlocked. Even though I bought an iPhone 5 from Verizon under contract, I was able to cut down my AT&T Micro SIM, and use it in my Verizon iPhone 5 to pick up an AT&T signal. By doing so, I was able to hop onto AT&T’s HPSA+ network, or “4G” as they so ridiculously name it.
As the report notes, users attempting to use a Verizon iPhone 5 on AT&T in the United States will not have access to AT&T's LTE network, due to hardware differences that leave the Verizon iPhone 5 (which is essentially a single CDMA/GSM world-mode phone for 3G purposes) incompatible with LTE frequency bands 4 and 17 used by AT&T and Canadian carriers.
For other international GSM carriers, Verizon iPhone 5 owners should simply be able to obtain a nano-SIM card from the international carrier and use it in their phone.