I've read The Hobbit several times before, including out loud to my kids only a few years ago. I don't think I'll need to read it again before the film.
The question is whether reading the book in advance will help to get more out of the film, or will lessen it. I think if you read the book a while ago and have a vague idea of what's going on then, either way, you should be OK.
I'm really looking forward to the film, but I'm surprised it's another three-parter released over several years. In comparison to LOTR, The Hobbit wasn't that complicated a story. I hope they're going to make lots of "episodes" that, on the DVD (or, by then, cloud) release, will be watchable as 10-20 minute chunks for bedtime viewing. The Hobbit is a story for kids, after all.