I've read some stuff on changes made to the GN for the screen, a few make me shake my head like what they did to the voice of Fate, but then other parts seemed to taken straight from the coimc. I had read that they changed the plot from the GN to make it out to be England was taken over by an outside facist government, but I don't know if that was true or not. V is a "terrorist" in the GN, but only in the sense that he blows up Gov't buldings with people in them, not blowing up dance clubs or buses or stuff like that. It seems they left that all in and didn't try to adjust it to the current situation, which would have ruined the story. Now I just hope they dont do a bunch of reworking to nerf it in light of the London bombings, since V bombs london himself in the story. The key they must NEVER show V's face. That was a central theme of the comic and if they have to give the actor some face time they are going to ruin it.
P.S. If you haven't read the GN I would, it's Moore's best work.