According to the Sci-Fi Wire:

In a statement, Warner said: "We have moved the release date of V For Vendetta to March 17, 2006, to accommodate the movie's post-production schedule."

The article continues to say that the spokesperson for the movie says the move has nothing to do with the recent London bombings.

Thanks to Trish for alerting me to this article. Right now Ain't-It-Cool News has posted the story, but it only parrots what was posted at The Sci-Fi Wire; likewise SuperHeryo Hype, but they do post a link to Box Office Mojo as confirmation of the change.

If more information surfaces, I'll be sure to post it.