While not bursting at the seems, this field has far more possibilities than the comedy/musical actress category.
1. Daniel Day Lewis, Nine
2. George Clooney, Up in The Air
4. Ryan Reynolds, The Proposal
5. Joseph Gordon-Levitt, (500) Days of Summer
6. Sacha Baron Cohen, Brüno
7. Robert Downey Jr, Sherlock Holmes
8. Michael Stuhlbarg, A Serious Man
9. Adam Sandler, Funny People
10. Woody Harrelson, Zombie Land
11. Bradley Cooper, The Hangover
12. Ricky Gervais, The Invention of Lying
I could see the Hollywood Foreign Press considering an inclusion of Steve Martin or Alec Baldwin here if they want to get really excited about It's Complicated, but chances are they will determine those to be supporting performances.
And there could be some An Education play in this category as well if it is determined that this drama is light enough to wedge into this category.