BetweentheFabric: If someone controls over 50 % of the computing power, they can (re-)write new blocks of history as they choose. Besides deciding what will and what won't make it into the blockchain (thus arbitrarily accepting or rejecting transactions), they could also double-spend their bitcoins.

The kind of risky investment bitcoins are at the moment, however, I guess most rich people have many more profitable options than paying a lot for rapidly outdating hardware to make a few bitcoin users angry.

Yours
David