You know, this adaptation could actually work, only because compared to the other popular animes, it's A) more realistic and B) it's not an action based shonen, so american directors don't have too try to hard, and just make a developing noir detective film with supernatural killings. It could be like SVU or something.
The only way Hollywood can mess this up is if they lie in the trailers. Death Note isn't (intenionally) funny, and Death Note had only one hand to hand match; IF Hollywood tries to even subtly pull this movie as different from the source, it will fail.
Now for the cast, I have two who could be Misa and Matsuda:
Reese Witherspoon: Give her the Legally Blonde Persona, long vibrant hair, and no brain, and you have the best portrayal of Misa right there. Couldn't find any good pic comparisons for them, but for Matsuda-
Jamie Kennedy in all his enthusiam would be a great Matsuda.
The last thing is, as long as the movie is just called "Death Note" it'll be good. You add "The Movie" or "Evolution" or any of that crap, the film will lose any possible seriousness.