Not necessarily. If the market is large enough and you control a significant portion of the hashing power (e.g. pool operators), you could bet below and then halt all hashing for the checking period. You would potentially lose a block, but would offset that by gaining more than 25 BTC in bets.
More importantly, if you have that much Gigahashing power, it's not worth manipulating this. You're better off sticking to mining Bitcoins.