A lot of it depends on what you are using it for.  If you are using it for commercial purposes then yes you absolutely need a license.  There is a test you have to take and I think there is even a small fee that needs to be paid to get the license in the state I live in.

If you are using it for personal reasons, I think the laws are a little more gray, but you are still encouraged to register your drone.

We have a couple of them at the school I work for and I had to give the teacher that runs the program information on how to get them licensed.  I am not sure if she ever followed through as I haven't seen much footage from them using them.