If this is truly an "Oprah Moment", then attendees will also have to pay tax on the phone just like any other prize:
http://money.cnn.com/2004/09/22/news/newsmakers/oprah_car_ta...
http://money.cnn.com/2004/09/22/news/newsmakers/oprah_car_ta...
In the US giving someone that does not work for you up to ~12k/year is tax free, but the person giving the gift needs to pay tax on gifts larger than that. With some paperwork there is a lifetime exception up to 1 million but it comes out of the estate tax exemption.
So, if Bill Gates gave his child 1 billion Bill would need to pay ~400 million to the IRS. However, there is an exception for anyone you are married to, and I think educational / heath care gifts are also excluded.
PS: I don't know if it counts as a "prize" or a "gift" if you hand it out to everyone that shows up, but I just wanted to make the difference clear.