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.
The idea that his pool is too large is not stupid, it is a valid concern. If you are worried about the security of bitcoin you should try to not put such a large area of trust in anyone's hands. While I agree it is not in Tycho's interest to do anything bad and really doubt he would..... an intrusion (attack) into his systems (online or physical) could effect the network.
So someone get's into his network. Say they manage to take every coin in the system and send it to themselves. Say they even manage to wipe out the logs files so it can't be tracked. It would still show in the block chain. If the pool operators didn't notice it, the users surely would when they weren't getting payouts, and pretty quickly inform the pool operators. Worst case scenario they halt all mining and go into investigation mode. People see that the pool is inaccessible and go elsewhere just like they did when slush's pool went down. There's a very limited time window for an attack like that. A more sophisticated approach would be to redirect the last 2 or 3 digits of the balance, but since people with updated clients can see that and all transactions are public it would be found out eventually. Sure people would freak out, but the network would endure.