[benmanns]

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.

[MichaelGG]

It's a daily bet on a weighted 12 hour average. So you'd probably need to lose much more than a block.