The idea Tycho would do something with his 'powers' as soon as his pool reaches 50% is just simply stupid.
Currently (in the current market) the guy makes 1000 - 1300$ PER DAY , that is 30k to 39k PER MONTH. In untaxed money even.
He would be stupid to do anything to destabilize the bitcoin market.
So what? There are lots of entities that would like to endanger Bitcoin and they would certainly bribe/infiltrate with money that would let your figures look like peanuts. But yeah, lets just wait until this happens and trust in Bitcoin vanishes in an instant. As long as you can get USD for your Bitcoins everythings fine.
Yes, there's a long theoretical list of people who might like to endanger Bitcoin. If any one of them have that much cash available to them then they can also build a super cluster and/or manipulate/corner the market. Both can be accomplished pretty easily and with relative anonymity for someone with said amount of cash. Trust would only vanish for people who are only worried about getting USD for their Bitcoins.
Based on the structure of bitcoin, wouldn't sheer processing power be the least profitable way to attempt to attack it? I'm more worried about tycho losing control of deepbit; the thought that entire this currency might hinge on one individual's security measures is a little disconcerting.