Please, whatever you do... DO NOT propose on the video board!!!
Proposal Fail « FAIL Blog: Pictures and Videos of Owned, Pwnd and Fail Moments
I would also advise against proposing on the big screen. I would bet 90% of girls would rather you do it elsewhere.
The chick who got proposed to was about 3 rows ahead of me at the game. She spent the rest of the time calling her mom and constantly trying to make out with her new fiance who was trying to watch the game. He looked annoyed, but in 2 years he will be begging her to look at him let alone touch him.