The Fisher PhD admissions process is rather lengthy. Across all programs (e.g. finance, accounting, marketing, HR, etc.), we will usually receive around 400 (or more) applications each year. There will likely be anywhere from 15 to 20 offers of admission out of ~400 applications. As you can imagine, the various PhD admissions committees take their time reviewing applications to ensure they are offering admission to the applicants who best fit the profile of admitted PhD students. Because of the volume and the extreme selectivity involved, this process can and does take time.
The majority of admission decisions for the PhD programs occur somewhere between late February and early April. If you apply with a complete application by the middle of December 2010, you should expect to hear from the PhD admissions committee no later than early April 2011 and probably between late February 2011 and late March 2011.