I believe that this 'Button' was given to all participants of the World Series of Poker 37th Annual Championship (Main) Event held at the Rio Casino, Las Vegas in 2006. They were all individually numbered.
The Reverse side states 'Milwaukees Best Light, ALL-IN' This was used by a player to indicate to all other players at the Poker Table that he was putting his Tournament Life 'on the line' and going ALL-IN. He would place the Button on the Reverse side, adjacent to all his/her Chips.