Straight from the horses mouth:
While we are conducting a 1 vs 100 US/Canada Beta Sweepstakes, please know that we are not awarding prizes as indicated during the game!
The reason we’re displaying the prizing indicators is that we need to be certain that the prizing components of the game actually work as designed. If we had pulled the prizing indicators for the Beta, we wouldn’t be testing the actual game.
For more information on prizing during the 1 vs 100 US/Canada Beta, please visit http://www.xbox.com/1vs100 and click Prizing Info.
Update 1st July 2009:
Speculation suggests in-game skill based prizes maybe coming to the beta test. All should be revealed in the next few days
Update July 4th 2009:
Chris Cashman announced in the latest live show that prizes will be awarded to the mob (not the crowd or the one) from Friday July 10th 2009
Update July 6th 2009:
In-game prizes for the UK beta begin on July 17th.