I don't know, but I'll tell you what I've found out. My corporation, PeopleCount.org, has received some gifts, and an accountant and someone at the IRS said there's no way to report it, so it doesn't need to be reported. It's not earned income, so it's not taxed.
My guess is that prize money is similar. If you ask the IRS, someone will get back to you. This isn't an official IRS policy-- they can decide otherwise later-- but it shows your due diligence. Similarly, you can ask an accountant and get her opinion.
If you have to work for the contest sponsor, and they receive your performance for the money, it could be argued that your company earned the reward, so it's not as clear as a straight donation. On the other hand, having income isn't that bad- you'll probably spend it pretty quickly. Most startups have money from investment with little or no income, so you won't have net income anyway.
I suggest you not worry about it, and if you win, spend $100 (or less) asking an accountant.