If you mean anything up to and including a 4S, no, you'll be able to activate it onto your (presumably) existing account by logging into the my account feature at vzw.com and choosing the 'activate / change device' option under 'manage my account.'
If you mean an iPhone 5, then you will need a SIM. You can go to your nearest VZW store and pick one up for free, or you can order one using the same process for switching a device (the website detects that you need a SIM and gives yout he option to order it when trying to add an LTE phone).