No, you can write about anything you want. The challenges/prompts are more something for people to fall back on if they have nothing to write about or need a bit of help getting the creative juices flowing.
i plan on participating...i made it almost three quarters through last year, and then had some pc issues that prevented me from writing for almost a week! once again, having issues, but this time internet related...so i am going to try my hardest! here's to a great NoJoMo!! hugs