From the FAA Website: " You will be subject to civil and criminal penalties if you meet the criteria to register an unmanned aircraft and do not register. "
Again, the Law defines Model Aircraft as follows (originally from the FAA website FAQ)
Is a UAS the same as a model aircraft?
Congress defined a "model aircraft" as a UAS that meets all of the following: Is capable of sustained flight in the atmosphere Is flown within visual line-of-sight of the person operating it Is flown for hobby or recreational purposes
We prefer to take a better safe than sorry approach to this issue. We'll be updating our member application to include your FAA registration number again as well as your AMA number.
The law doesn't require you to register every plane/heli/drone you own. Only that you register yourself and affix the FAA number somewhere on your aircraft. It can be hidden inside the battery compartment as long as the number can be easily accessed with simple tools - like a screwdriver.
If you haven't already registered, you can do so here: https://registermyuas.faa.gov/register
The AMA has created a small FAQ to answer questions about this matter, and you can find it here: http://www.modelaircraft.org/aboutama/faa-uas-faq.aspx