Even miners controlling large amounts of hash power are significantly limited in their ability to unfairly benefit from their hash power:

Miners can't:

- spend other people's coins
- issue themselves 'extra' bitcoins (beyond the block reward)
- spend coins twice (in a permanent way such that extra coins come into circulation)

Sure they can attempt to "double spend" and be guaranteed to succeed with a certain probability, but this just means they tricked someone into thinking--for only a very short amount of time--that they were paid when in fact they weren't.  When the double spend is complete, everyone will see that the coins were only ever "really spent" once.  This is why you can't withdraw your 1000 BTC jackpot from just-dice.com until your original deposit reaches 7 confirmations.