...I wonder if there'll be a Rule 4!?
Hot dog-devouring legend Kobayashi to miss Independence Day dog fest - CNN.com
They'll take their lumps like a good 'un, make the market void or the bookies will be hoping that Joey Chestnut chokes.
EDIT: Nope.
Bet365 states that
'Both Chestnut and Kobayashi must take part in the competition or else no action. Win only.'
Boylesports and
Ladbrokes state no such stipulation and they offer 4/11 on Joey Chestnut whilst still quoting Takeru Kobayashi at 7/4.