Milk is the only film nominated for both Picture and Original Screenplay and it won the WGA for Original Screenplay. Both of these things work in its favor but, at the same time, it seems like the buzz for the film in generally has steadily diminished since its release. Add to that the fact that the screenplay, which hangs on the tried and true tentpoles of the biopic genre, is probably the weakest part of the film and it's easy to see how and why AMPAS might vote another way.
Hunt first made Frozen River as a short film in 2004, which she shopped around at various festivals over the course of a couple of years in order to get financial backing to turn it into a feature length film. She was eventually able to secure just under $1 million to shoot the film on location in 24 days. Like Ben Affleck and Matt Damon or Diablo Cody before her, the story of how her film made it to theatres is just as compelling as the product up there on the screen - which is why I think she might just walk away with the prize.
Leigh has been here before. He was nominated for writing Secrets & Lies, Topsy-Turvy and Vera Drake and walked away empty handed each time. I think the legend of his working style, which involves a large amount of improvisation from the actors, really works against him because it naturally implies that he had a smaller hand in actually writing the screenplay. Of the nominees, I think he's the least likely to win but I'm glad that the film got nominated for something, since it was one of my favourites of 2008.
It's a testament to the strength of the film that it managed to stay on the radar long enough to secure a nomination, given that it was released in theatres nearly a year ago. I don't think it'll win, although you never know and the UK contingent of the AMPAS might push it over the edge. This is McDonagh's first nomination as a writer, though he does already have an Oscar for Best Live Action Short, which he won in 2006.
The AMPAS has nominated Pixar films on a number of occassions (Toy Story, Finding Nemo, The Incredibles and Ratatouille all received screenplay nods), but none have ever won, which may be attributable to the fact that, at the end of the day, animated films are still largely considered "kids stuff" and not taken as seriously as they ought to be. Stanton has been nominated twice before (for Toy Story and Finding Nemo), Doctor has been nominated once (Toy Story), and this is the first nomination for Reardon.