No, it does not. You must requalify before Dec 31.
it is just that your status is valid till March of the next year. Most airlines do something like that. Because a flight taken in late december will take time to be credited (e;g; it could be missing and you claim credit by mail or fax), Fb waits a while to do its final accoutning of all the miles you earned in 2008. But no flights taken after Jan 1, 2009 will count toward your status earned in 2008.