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.
...many online exchanges only require 8 or 10 confirmations before funds can be traded. In that time, you could literally crash the market with your double-spent funds. Then just as people figure out what you've done, you buy back in as the market is recovering.
Or maybe I'm missing something that prevents all of this... if so, please tell me.I think one thing you're missing is the fact that the nefarious miner can only succeed with a certain probability. Consider a nefarious miner with 25% of the global hash power:
The probability that he mines the next block = 25%
The probability that he mines the next two blocks is 0.25 x 0.25 = 6.25%
...
The probability that he mines the next six blocks is 0.25^6 = 0.024%
The probability that he mines the next seven block is 0.25^7 = 0.0061%
Consider your example of double-spending to crash the market: the nefarious miner transfers 10,000 BTC to MtGox to dump, and then starts trying to mine a new chain fast enough that he can "undo" this 10,000 BTC transaction. While feverishly mining, he waits till his MtGox deposit has confirmed, and then market sells his 10,000 BTC. Due to slippage he gets significantly below market price. Then it dawns on him that since he only has 25% of the global hash power, the chances that he will actually succeed in this double-spend attempt is remarkably small. He literally must perform this fraud attempt hundreds of times before he is likely to succeed. Each time he fails, he looses a significant amount of his capital (because he just did something stupid like market selling 10,000 coins). In the extremely unlikely event that he succeeds before he runs out of bitcoins, what he did will be pretty obvious since he would orphan a long valid chain, that, hmm, just happens to correspond with the big dump at MtGox.