In July 2010, the FAA published a final rule that establishes specific registration expiration dates over a three-year period for all U.S. civil aircraft registered before October 1, 2010, and requires re-registration of those aircraft according to a specific schedule.
Currently, aircraft registered in July of any year will have their registration expire on March 31, 2012 and must apply for re-registration no later than January 31, 2012. Owners of aircraft with a July registration should have already received an expiration notice that contains an online re-registration code.
To ensure the FAA has the correct address information for your aircraft, you can check your N-number here: http://registry.faa.gov/aircraftinquiry/nnum_inquiry.aspx.
An FAQ and complete schedule of expiration dates can be found here: www.faa.gov/licenses_certificates/aircraft_certification/aircraft_registry/reregistration_faq/