There is a new requirement (2010) for aircraft to be re-registered every three years. The FAA will mail registration documents to the address they have on file for you—unless they’ve mailed something and it’s been returned. The FAA will cancel the N-numbers of aircraft that are not re-registered or renewed. The re-registration fee for the first round will was $5.00. And it remained $5 for the second round.
Several people I know have had their registrations expire because they moved and neglected to update the address for their airplane. A quick and simple process turned into a months-long wait for their new registration. And of course, you can’t fly if your plane is not registered. And not only that, but your insurance is probably not valid either because most policies require that the aircraft be airworthy to be insured, and without a registration, you aren’t airworthy.
14 CFR §47.40 Registration expiration and renewal.
(3) A Certificate of Aircraft Registration issued under this paragraph expires three years after the last day of the month in which it is issued.