Your Gold card, when issued will have an expiration date on it. normally February of the next year. I would assume that making it in December 2010 would give you until February 2012. Becoming Gold in January 2011 should have the same effect, assuming you have enough nights from rollover and stays within the month of January to hit 50 nights.
Here's the official language from Marriott.com:
To achieve or renew Elite Membership status, the following number of personal nights must be earned through Rewarding Events or stayed at participating locations between January 1 and December 31 of any given year:
A call to Marriott Rewards on Monday might clarify the issue. And good luck.
I'd call Marriott Rewards and get the exact count--while there is some overlap--the balance does not go to zero on the first of the year but several days after the first--there is an annual stay requirement that coincides with the calendar years (e.g., Jan 1-Dec 31).
So, in essence only calendar year 2011 stays and bonus nights (from the Premier Marriott Rewards Chase Credit Card if you have one) count as Elite Qualifying Nights to make an Elite Level again for 2012. If you reach a Gold or Platinum (or even Silver) level during 2011, you get that status as soon as the nights are posted to your account for the remainder of the year and for 2012 as well.