Yeah, I'm running Nitrogen in my tires. I'm doing it more for the tire life itself - Nitrogen has larger molecules, and won't leak out as fast, and also it is an inert gas, that will help the tires from becoming weatherchecked with age - or at least that's what "they" say when trying to sell it.
Whether or not it helps fuel mileage, I dunno...
EDIT: Oh, and I always get tickled when the comparisons are done with Oxygen and Nitrogen. The air we breathe is already 78 percent Nitrogen and 20 percent Oxygen, so I'm not sure why comparing with Oxygen is the way to go: I sure wouldn't put pure Oxygen in my tires!!!