Given that Alan Moore has gone out of his way to say he's had nothing to do with the movie, I'm not sure quite what it's going to be like. Perhaps if you've not read it, you shouldn't do until you see the film - that way you can't be dissapointed. =/
Ah yeah - the release date is sometime in March, I think. I was reading a post from someone elsewhere suggesting that in keeping with all the changes they were apparently making to the storyline, they should also change the tagline from "Remember remember, the 5th of November" to "Beware the Ides of March."
