As of this posting, NASA has NOT posted data for December 2008 nor have they released their formal assessment of the past year. The reason can be found here, with the relevant passage being
Starting this month, the data will be held, investigated, and potential problems reported to and resolved with the data provider before making them public. However, as we noted in the "Data Quality Control" section of our 1999 paper: We would welcome feedback from users on any specific data in this record.
Given how politically charged each data point is (something I believe they failed to recognize earlier), I think this is a wise move. There's no hurry. Get it right first.
BTW, although they didn't publish a formal assessment of 2008 temperatures, they do have a preliminary assessment. They note that this past year has been the coolest since 2000, but still ranks as the 9th warmest year since instrument records were first kept. Global Warming deniers will give you half of this information. Care to guess which half?