JayK: don't know, but if there's ever been a graphically illustrated story that needed to be made into a movie, it's The Watchmen.
Never read Walking Dead, but I have the first 45-or-so issues of 100 Bullets, and it's 99.9% as good as Sin City (only because Sin City came first). Fucking brilliant series of stories and great running theme.
Another brilliant story that would make the transition to big screen rather nicely IMO is a DC Vertigo title called "Y, The Last Man." It's basically about a guy who quite literally is the last man on Earth after a strange plague wipes out everyone with an X chromosome...except him for some reason (thus the book being titled "Y..."). You can imagine what happens when women start to discover that there IS a man left. Shit gets hairy right quick.