A lot of people frown on the use of iPods during the official race. I didn't bring mine last year mostly because during my long runs I'd found it distracting during the tough spots when I needed to focus. However, by mile 18 or so I wished I'd had it with me. Not for any other reason than by that time I was getting tired of hearing people yell. Maybe this is a New Yorker thing, but by the time I reached the First Avenue stretch, I just wanted everyone to shut up for a while so I could refocus, something that could have been more easily achieved with my iPod. Unfortunately, unlike some other stretches (parts of Bedford Ave. and portions of the Bronx), First is packed with people. If you are in the later waves, it's likely many of these people will be drunk. So, something to think about. Even if you just use it to give yourself a break from the cacophony.
Related: Your iPhone. Even if you live here, you will want to take pictures, especially along the 4th Avenue stretch where the mighty Brooklyn roars loudest (and you are still relatively full of energy) and then along the 59th Street Bridge, which offers breathtaking views of the city.