The Oscar for Best Film Editing is probably the most important Oscar to night, because if he wins Slumdog millonaire and will probably leave intact their aspirations to win the Oscar for Best Picture, but if he wins The Curious Case of Benjamin Button will be probably the same film won the Oscar for Best Picture.
The Dark Knight has a great assembly and winning it would not be surprising for the high technical quality of the film, while taking into account how vital it can be this Oscar for Best Editing in the struggle to win the night, it would be rare to win. If you win, I would say that among academics there is no really truly favorite movie and the chances of winning Slumdog millonaire or The Curious Case of Benjamin Button would be 50%.
It also is Milk, which do not have many options in this Oscar for Best Editing and no a priori tine no chance of winning the Oscar for Best Picture but it won the Oscar for Best Editing, traditionally two of the important next Oscar for Best Screenplay, would have serious choices to win the Oscar for Best Picture and would become the favorite. Although this does not happen I think.
Frost/Nixon has no chance of winning the Oscar for Best Editing.