Basically, what you're talking about here is what the IRS would consider a form of gambling, unless this was something akin to a raffle for a charity or something of that nature.
I've looked for the IRS publication on this, and I can't find it. I recall reading about this in some obscure IRS pub years ago, and for all I know the rules/laws could have changed since then. But in order to provide you "something" to work with, I'll assume there's been no change.
Basically, if you "played the game", then as far as the IRS is concerned you played with the "intent" to win. So weather you accept the prize or not doesn't matter. This basically comes into play only if you are sent a tax reporting document for the prize. So if you received a 1099-MISC or some other tax reporting document, it's reportable on your tax return weather you actually accepted the prize or not. There are caveats to this though. For example, if you declined the prize and they awarded it to the 2nd place winner.
