I suppose the key figures are Rorschach, Nite Owl, Ozymandias and Dr Manhattan. The Comedian is only seen in flashback, but he's important to get right. Also the two women, Silk Spectre I and II.
Bear in mind I forget how to spell all famous names.
David Hayter is down as affirming that he would not consider David Caruso as Rorschach, despite the strong physical resemblance between the actor and the character. Somehow I quite like the idea though. My second choice would be Ed Norton, I think. This is a great role, and there's a whole bunch of skinny, screwed up, American actors who could be great in the role.
Nite Owl. I read somebody's suggestion that this character should be played by Tim Robbins, and I like it. I can't think of a better casting.
Ozymandias. Aryan, buff and left wing, with a slight tendency to insanity. I'm thinking Woody Harrelson here. Christian Bale might be good too.
The Comedian. I would love to see this role played by Steve Martin, but I can't see him agreeing to play a rapist and murderer. Willem Dafoe probably would though.
Silk Spectre #2 - Carrie Anne Moss? Too cliched? But fits the role well.
SS #1 - I'd like to cast Susan Sarandon, but she is a bit too young, and a lot too pretty. Still, love to see her in anything, so she's on my dream cast.
Dr Manhattan is quite hard to cast. Big 'n' blue 'n' bare. Doesn't get old. Perfect features. Somewhat god-like and disengaged. I'd say Harrison Ford but he's too damn old now. How about Jude Law? I know he isn't American, but he can do a decent stab at the accent, and he's prepared to get his kit off. But is he quite butch enough? Other than him I'm stumped.
If anyone has any other suggestions on any of these I'm happy to hear 'em, and I'm going to ask on some of my mailing lists too.