It looks like we're 2 for 2 when it comes to unlocked iPhones on Verizon's network. As you all know, I picked up my Verizon iPhone 5s on launch day, but I haven't been able to test it out with a GSM sim card until today.
I walked into Walgreens just a few hours ago, and picked up one of T-Mobile's No Annual Contract kits that contains the SIM card starter kit. Unfortunately the SIM card that came with the kit was one of the old-school normal sized SIM cards (people still use those?), so I had to break out a pair of scissors and cut it down.
After a few minutes of manicuring the SIM card down to a nano size, I was ready to rock and roll. I popped out my Verizon SIM, inserted my T-Mobile SIM, and sure enough, I was on T-Mobile's "4G" network.