I have been testing out the Verizon iPhone 6 with other carriers. While I am not sure if early adopters of the phone will need to have them unlocked prior to the law changes in the U.S. (I think back in November 2014), but I do know that the phone works flawlessly with T-Mobile (4G LTE) and AT&T.
Test #1 Used a T-Mobile 4G LTE Hotspot SIM (Just Data) and mapped it to the IMEI number on a IPhone 6. Test was successful, but couldn't make calls since it was initially setup for data only and obviously mapped to another IMEI.
Test #2 I purchased a AT&T SIM for the iPhone and mapped it to the IMEI number on their website. I turned off the phone, inserted the SIM, powered up the phone and boom, got service and text message from AT&T. I had previously requested to unlock my phone prior to this, so I am not sure if unlocking is required first. When trying to insert the SIM without powering down the device first, the phone did not recognize it when performing my test.
Test #3: I purchased an AT&T SIM to use in my T-Mobile Samsung Hotspot Pro. I did call the company to have the T-Mobile device unlocked prior to changing the SIM (you have to have an active account to unlock the device). Initially, I couldn't get service just by inserting the SIM, I had to map the SIM to the devices IMEI number on the AT&T website first and apply GoPhone funds to the account. It takes about 10-15 minutes before the signal initiated. This works great now, just more expensive than T-Mobile.
My understanding is that all 4G LTE by law cannot be locked, but I do believe 2g and 3g may still be locked on some phones with certain carriers.
Verizons Unlocking Policy: