Let's see.
The Comedian is shown in the movie from his 20s to his 60s, with most of the scenes when he's 40. So an actor in his 40s, like George Clooney, who probably would love to act a bastard republican superhero.
Dan Dreiberg in the script is someone in his late 30s, and I'm thinking that Ed Norton would be better in this role than as Rorshach.
Laurie is in her early 30s with a flashback showing her at 18. Hilary Swank was attached to the role last year, and I think she would be great.
Jude Law wanted to be Rorschach (thanks to heavy make-up) but he would be a great Adrian Veidt.
Paddy Considine for Rorschach? He looks the part and he's a good actor (see "In America" if you haven't). Too bad that he's 5ft11, a bit too tall. Someone somewhere suggested Sean Penn as Rorshach, and that's not hal a bad idea.
For Doc Manhattan, they're going with a painted actor, there's no reason for him to be CGI, and we need someone tall and with a great body. Eric Bana or Gerard Butler seem good candidate.
And Cristopher Plummer for Hollis Mason.