Love the movie; love its message. Never read the book, but from what I understand about it, the movie was probably a big improvement.
I see nothing wrong with "Behind this mask is an idea, and ideas are bulletproof."
The V masks everyone is wearing stems from the whole anonymous movement - the OWS protesters didn't start that. I see it as a completely valid form of solidarity that is symbolic of what happens with the crowd at the end of the movie, not necessarily having any ties to the action V takes.
Some people may be crazy and do crazy things, but they are ultimately the problem, not the movie that influenced them.