What Should and Will Win: Milk (written by Dustin Lance Black)
In the sharpest screenwriting debut of the year, Black brings to life an icon, and with great dialogue. I know the Academy feels the same, and this should be a part of Milk's mini-sweep. Van Sant and Penn did masterful jobs as well, but Black really masters the movie here, and he will definitely be around for a long time.
The other nominees:
Happy-Go-Lucky (written by Mike Leigh)
What a wonderful movie and screenplay this is! Leigh provides the sparks that light the fires of Sally Hawkins and Eddie Marsan, both who branch fantastically off of it. Leigh does his absolute best here, a magical piece not only in his direction but in his writing.
In Bruges (written by Martin McDonagh)
I believe this may be the funniest script of the year. I laughed a few times, and that's not easy to make me do that. Of course, its only embellished by Farrell and Gleeson, but "Six Shooter"'s lauded director creates the one of the most unique films of the year, running off a mostly one-joke enterprise (however, it's hilarious).
Wall-E (written by Andrew Stanton and Jim Reardon, story by Andrew Stanton and Pete Docter)
Stanton does his best work with Wall-E, the best Pixar film, one where I didn't groan all the way through do to "genius" being coasted off of. He gives a message-filled idea some humor, life, and originality. The scarcity of dialogue only makes it better.
Frozen River (written by Courtney Hunt)
Yes, it's a good script and all. But, no, it couldn't have worked without the exceptional work by Melissa Leo. Hunt's somewhat generic work basically falls through without the support it gets. It's still good, though, and has that predictable-yet-surprising nature that I described in my review.